"(1) It just turned out that when Google was started, the core programmers were C++ programmers and they were very effective. Part of it is a little bit of culture. (2) Early Lisp programmers (Erann Gat) at Google actually noticed that other programmers were equally or more productive. It has more to do with the programmer; we're getting to the point where language choice is less important (as opposed to 20 years ago). (3) Lisp is optimized for a single programmer or a small group of programmers doing exploratory work... If I want to make a change in a weekend I'd rather do it in Lisp than anything else, but by the time you get up to hundreds of programers making changes are not a language problem but a social one. (4) Libraries."
Paraphrased from: http://www.youtube.com/watch?v=hE7k0_9k0VA#t=03m20s.