I hope we are talking about the same type of "why." By "why" I do not mean the "reason for", but rather the "cause of."
From what I can observe in the world around me, I can see that all things happen in a cause-effect sort of manner. It is my thinking that if the Universe had a beginning, it must have had just such a beginning, i.e., cause-effect. However, then we must find the cause, and if the cause is found, then we must consider that the cause is actually
part of the Universe itself, and that the cause (the Universe at that time) only added to its size by creating the effect. In this case, we must find the cause for the cause, which now becomes our new effect. This endless "tantrum" could go on forever!
However, if the Universe were to have always existed, then it shouldn't be moving from a centre. Because if we do rewind the tape, the Universe can become only so compact in that small space. From this, we can conclude that the Universe (at least as we see it) had a beginning.
Or, we can conclude that the Universe existed in that state for an infinate period of time, and only recently did it "bang" outward. Once again, we see that there ought be a cause to this "bang." Which leads us to searching for yet another cause, leading us back to the "tantrum."
So, there must have been a beginning, and there must have been a cause.
The Big Bang focuses on the cause of this beginning, and the explosion for which we have no proof, is really no more a valid argument than an intelligent creator for whom we have no proof. Science is supposed to base its conclusion on evidence, yet, it has come to the conclusion of the Big Bang, with little to none for its support.
Trék