Ha.
I meant before, isn't it weird that you can't Google this and there isn't a prominent page with a substantiated answer? You'd think this is a question that a majority of people on the internet would have wondered about at some time or another.
RE: culture et al., I really think that's not true. People are people, everywhere. The mitigating circumstances in development always resolve back to extra-human things-- like climate and conditions, not "x, y, z advantageous value" held dear to the hearts of certain societies. I'm not saying those things don't matter, just that they ALMOST always don't.

Culture invariably is NOT as big a deal as people think. But that's just the point: we think it. If you ask most Chinese today for an answer to this population question, they'll say "Chairman Mao and the Cultural Revolution"-- which is completely wrong but, hey, that's what they _think_. And in my experience, "culture" influences much more what people say/think than what they do.
And India is also huge, but nobody would make the argument that it's because India had historically-minimal plagues (*cough* malaria) and that Indian states have enjoyed exceptional life expectancies and very peaceful histories, or that India -- crossroads of the damn world -- was particularly isolationist. Quite the opposite on all counts.
No, this looks to me like a case of "different initial conditions", which is to say, much earlier settlements than elsewhere in the world. But, the archaeological record as it exists today doesn't really bear that out.
Quite odd.