Occam's razor. A perpetual universe is only required to exist - which also happens to be all we can 100% empirically confirm, whereas the other explanations require creating extra complexity such as a birth/creator or before/after.
Even a before/after of "nothing" is still an extra complication that violates the principle. Not that Occam's razor is a law, but given something already as wieldy as everything, it's probably a prudent application.
A perpetual universe has it's own set of very complicated problems, resetting entropy being the largest elephant in the room. Occam's razor is still twisted into nth dimensional shapes no matter what choice you make here.
There are no easy answers, and it's highly likely whatever answer is true is unknownable.
Occam's razor works when the two alternatives are equally likely. In our case, we have the ratio of primordial elements (which is well explained by the big bang) and the cosmic background radiation (which is extremely well explained by the universe coming out of a hot, fiery early stage, within an error of 1/1000th).
I.e., a perpetual universe must explain why in every direction we see a uniform background radiation which looks exactly like it's coming from a hot universe of the past. Occam's razor dictates that the big bang is the simpler (thus preferable) model, given evidences.
If our view of the universe shows an infinitely compressed point of matter expanding, what can coax such expansion? An infinite surround of nothingness. Which must then be contained within what? More matter; endless ripples.
As another commenter posited, a space-time surface that is curved along itself, perpetually. It doesn't negate the big bang, or the great death, rather it relies on these two properties - but exclusively and reductively.
The original comment was regarding a cyclic universe - which I agreed with. Perhaps my use of the term "perpetual", while accurate - was confusing. I don't disagree with the big bang, we've observed it directly.
In the same way that the two-dimensional surface of a sphere has a finite extent, it's possible to imagine spacetime as a 4-dimensional surface in which the time dimension is finite in extent. In this way, "before the beginning" has no meaning in the same way that "north of the north pole" has no meaning.
Even a before/after of "nothing" is still an extra complication that violates the principle. Not that Occam's razor is a law, but given something already as wieldy as everything, it's probably a prudent application.