In order to properly frame the question, we must remember that, at this very moment, there are non-existent philosophers in a non-existent universe asking themselves why there is nothing rather than something.
In order to get a more serious answer, I think we need to try rewording the question. First attempt: "What caused there to be something rather than nothing?"
I think that as we peel back the cosmological onion, it becomes difficult to meaningfully speak about "causes". I recognize this is somewhat of an evasion, and it is admittedly a deeply unsatisfying answer. But it may be the most technically accurate answer. Take multiverse theory: It may turn out to be ridiculous to try to apply the concept of cause-and-effect outside of any given universe -- in which case, asking "What caused the multiverse to exist?" would be one of those phrases that is syntactically correct but semantically meaningless.
Because this is so unsatisfying, let's try one more attempt at rewording the question: "What is the 'nothing' that caused there to be something?"
The Ultimate Answer is below the fold.