Vlad the Beggingthequestionist,
Depends what you meant by "why". If you meant, "by what process?" or "how?" then quantum fluctuation is the most developed potential explanation so far, albeit a far from complete or tested one.
If on the other hand you meant by it, "for what purpose?" or some such, that's called begging the question because you'd need to demonstrate first an agency to decide that and then to carry it out. It's the same mistake you made a few posts back when you complained that irrealism doesn't produce "unequivocal" moral positions. You'd need to demonstrate first that moral statements would have to be unequivocal to be "real", rather than just provisional conclusions based on the instincts and reasoning available to us.
Another fail. Why do you bother?
1:Asking by what process is no answer to the question ''why something and not nothing'' because even a process is a something and a something is not a nothing.
2: On the other hand for what purpose is a red herring on your part because there doesn't need to be a purpose does there?
But that needn't mean no purpose. The question ''why something and not nothing'' is a question any persons, theist or atheist,teleologist or ateleologist can posit.
There is no record of Krauss' pop science book being the thing which established either the process or the purposelessness of the universe popping out of nothing and certainly no method to test the idea.
A process is a something which brings us winging our way back to the question ''Why something and not nothing''?
But don't take my words for it there are scientific American blogs attacking the claims of Krauss and the endorsements of Dawkins and on
www.Nauti.US on Krauss and the change the meaning of ''Nothing''.
I will present them presently.
Finally I credit Krauss with trying to have a cake and eat it. In other words reaching for a verbal formulation where the universe both pops out of nothing and is around for ever.