Why is there X where X = (Existence != IS).
As I have said in many answers, Existence does not equal IS. The normal formulation of this question is Why is there something rather than nothing which is called the ultimate question. But to ask why exists X and why is there X are two very different questions, not the same question as you have implied ranking them together as if they were synonymous.
One interesting thing is that there is no meaning to Existence, as existentialists continually point out. Being on the other hand equals intelligibility or meaning as Parmenides said “it is the same to think and to be”.
Saying Why there exists something means why do I find myself thrown in the midst of things within a world as a thing myself. This according to Heidegger is an existentiell question which is discovered (befindlichkeit). it is part of the nature of dasein to ecstatically project the world and itself as part of the world. It actually has no why per se rather it is a facticity.
On the other hand Why there IS something is tantamount to asking about the intelligibility of what “is” and the meta-question concerns the meaning of Being. Is “IS” a facet of Indo-european language in particular and is part of our “rede” talk another existentiell. But also IS confers understanding within our particular worldview and thus gives rise to our unique kind of “verstehan” understanding another existentiell according to Heidegger.
So it is interesting that it is only one existentiell that relates to existence while the other two relate to Being per se as linguistic intelligibility peculiar to the Indo-Europeans.
This is probably not something that Heidegger himself would agree with but it reinforces the point that for the Arabs wajud (existence) was part of Being (kun) but not all of it. A rock exists, but it does not project a world which it can discover itself to be part of. So to the extent that we can discover ourselves to be part of the ultimate horizon of world then we exist as an ecstasy, but to the extent that we can comprehend and talk about that world overflows from the ecstasy alone into the projection of meaning or intelligibility.
For Heidegger the key point is that these existentiels fuse together at the core of dasein as being-in-the-world as “Care” (Sorge). In other words the discoveredness of oneself within the world already as part of the ecstasy of projection is the same in some way as understanding it and being able to talk about it.
We can turn this around and say that because Indo-European language has Being, that affects the intelligibility of everything within that worldview and takes us beyond what we would have if we only had the standing of existence or the copula or some other central concept within our language, as other languages have. So there is a special kind of intelligibility that comes along with the concept of Being, and that intelligibility is expressed in our language and in our understanding which is unique. That uniqueness has to do with the way we use tropes like Metaphor, Metonymy, Synecdoche, and Irony. Other languages produce tropes by juxtaposition, while we use tropes by creating a substrate of connection through the projection of the illusory continuity of Being. In existence things stand on their own while in Being things stand through each other in an odd way. So Odysseus IS a lion which is more than the simile that could be had by juxtaposing Odysseus with Lion and allowing us to infer their similarity. Odysseus IS a lion means that somehow Odysseus and the Lion share the same substance.
So if we come back to the question Why is there an IS? the answer is we do not know why Indo-European has this odd yet unique peculiarity. But by the Worfian hypothesis we can infer that it must effect our understanding of the world when juxtaposed with the existence, copula or other basic concepts that are standings in other languages. Existence is a standing toward what is found (befindlichkeit) by finding ourselves already in the world. Copula (such as the Me in Sumerian) which is a unique agglutinative language is a standing of things toward each other. Interestingly the term Me also means something similar to the Tattva in Tamil which gives us the idea of dharmas in Buddhism. In this kind of language existence is fragmented into little mechanisms that just work, and we don’t know why but they are there in an array of juxtapositions in existence. On the other hand existence, like Wajud in Arabic is unified through its ecstasy that allows that which is to be found to be found.
Why there IS an “is” is unknown. But we do know it is a uniquely Indo-European concept that has a peculiar linguistic form in all related languages of that family. We also know that it is a construct because it is forged from different linguistic roots within proto-Indoeuropean, which is to say it is an artificially fabricated cultural construct, not something that arose naturally within the language, because it is constructed of disparate linguistic elements. And what we notice about the Indo-Europeans is that they were the first to domesticate the horse, and use that to conquer the unknown world making it known to them as their possession. It turns out that “having” is an equally fragmented linguistic root within Indo-European languages. Being and Having go together in some fundamental way in the Indo-European worldview. It was a linguistic project to forge Being/Having within this linguistic family, but why this is true we don’t know. But what we do know is that this has to do somehow with our ability to produce technology and that has something to do with the rife nihilism of our worldivew, at least according to Heidegger, and to some extent Nietzsche.
On the other hand, existence or copula has no why, in the same way as Indo-european has a why. In other words, without Being why itself does not have the same force. Within our language we have the fundamental structure Who, What, Where, When, Why and How, which in different ways probably exist in other languages as prominent. But after the ultimate question these are the subsidiary questions. And it is the Why that we associate with Being, because Being is for us the same as intelligibility and intelligibility is synonymous with knowing why.
If we ask Why is Being, then we get the answer of God’s will, or destiny, or fatedness (dreeing the wyrd)
If we ask Who is Being, then we get the answer God (Supreme Being) who is seen as Good only with evil a mere privation.
If we ask What is Being then we get the answer a substance that translates into omniscience of God which betokens the Rightness (RTA) of the Justice of God.
If we ask When is Being we get the answer the Hence/Thence (Now/Then) (Ongoing/Completed) which translates as the eternality of God which betokens the perfect order of Gods law beyond our understanding of the accidents of human life.
If we ask Where is Being we get the answer Hither/Thither (Here/There) which translates into the omnipresence of God which betokens the World as the dual of logos/physos, and the word that becomes flesh as an avatar that unites these duals. (omnipresence suggests omni-identity, omni-reality, and omni-truth, i.e. God is One, God is Reality, God is Truth, i.e. God is aspectival).
If we ask the How of Being we get the answer of omnipotence of God which betokens trinitarian mystery and the three fold nature of the existentiels.
This suggests that the subsidiary questions to the ultimate question has to do with the qualities of God as Supreme Being under the auspices of ontotheological metaphysics and that these are in turn related somehow to the nonduals at the core of the Western worldview, i.e. fate, good, right, order, etc.
The tropes are Metaphor, Metonymy, Synecdoche, and Irony:
God IS Being, Holy Ghost is God, Son IS Father, The three are one.
All this is OntoTheological Metaphysics according to Heidegger which is a metaphysics of Presence that discounts Absence, a metaphysics of Identity that discounts difference, a metaphysics of Truth that discounts Fiction (Lies), and a metaphysics of Reality that discounts Illusion or unreality. To the extent that we pay homage to formal systems and also are realists we continue this tradition. Formal systems are based on Truth, Identity and Presence aspects whose relations give the properties of completeness, consistency and clarity (well-formedness). When we add in the Aspect of Reality then oddly enough we generate meaning and also the properties of Verifiability, Validity, and Coherence. Together the properties of the formal system can give soundness but we need the properties that associate the formal system with reality to give correctness.
Peirce contributed the Existential Operator (Backwards E) to logic. Without that our logic floats free without any tie to actual facts. Logic itself is composed of three operations And, Or, Not. The other important operator that balances existence is All (upside down A) that differentiates the Universal from the Particular. The Universal indicates the Set Like differentiation of particulars. While Existence is Mass-like and indicates that there is a dual to sets which are Masses, and a dual to syllogistic logic which is the boundary or pervasion logics. Our culture forgets the non-count and emphasizes the count and thus we base our math on Sets, and our logic is syllogistic. We forget about non-count masses (like furniture) and pervasion or boundary logic (like Laws of Form, G. Spencer Brown). Masses and Boundary logic is more related to Existence, while Sets and Syllogistic Logic is more related to Being.
Thus the forgetting of Being is at the same time a remembering of Syllogism, and Sets and the Oblivion of non-Being as existence along with Masses and Pervasion Logic as the dual of sets and syllogism. One reason we cannot think Existence is because we have lost the tools of thought, and this is part of the OntoTheology of our worldview. Existence appears in our worldview as the dragon (typhoon, python) destroyed by St. George (Zeus, Apollo). This triumph of Being over Existence comes at a cost, one of which is our disconnection from the planet on which we live (unfortunately at this time as parasites). The benefit is the ability to integrate technologies that other cultures have not been able to do. No one knows who invented the wheel but Indo-Europeans invented the Chariot as the first war machine, when horses were big enough to pull them in a team, but not yet big enough to ride. This was the beginning of the first great colonial wave of Indo-European domination, the second wave of which was the colonization of the whole world by sea power and gun powder, and the third wave is globalization of Indo-European culture happening now.