Firstly, and I'm not assuming that you intend a creator god to be the preferable explanation (though of course many do), the same question can be applied to that god. If one always existed, he/she just sat there for an eternity before acting. Why?
Secondly, models of an open, non-oscillating universe generally imply that time as we know it did not exist before (or as one apologist put it, "ontologically prior to") the Big Bang. As one part of Einstein's theoretical combination "spacetime", it was wrapped up with everything else in the singularity (speck) and its apparent forward flow began when the expansion did. Going by this type of model, it wasn't possible for anything to happen "before" the Big Bang. Stephen Hawking likened the question to asking what's north of the North Pole.
Thirdly, it's possible that there are systems of time and space outside the universe we know, and our universe was set off by one of these. There are various models of a multiverse, collectively infinite in both time and matter, where universes regularly beget other universes. There are also slightly simpler models of a single external mother universe or "metaverse" which spawns the others. These are speculative, of course, but at least the extra entities being posited are objects we know exist in at least one case, i.e. universes.
Finally, new evidence is emerging which may challenge the singularity's perceived lack of a beginning. It may be best to wait and see.