This is a good question. I'm thinking it's because nomads and so forth ventured out here later and thus devoloped slower. Everything started over in the Middle Eastern area and then to branch out and devolop into what are now the European and Indo-Chinese areas. So, when they crossed the Berring Strait the nomads had such a broad area to cover they just kinda pushed on and didn't set up shop like those in Europe, the Middle East and China did. Thats what common sense is telling me. I could be dead wrong.
I have similar thoughts. Huge mass of land, hard to civilize if you keep migrating from place to place for food, shelter. Different rate of births possibly due to culture. Also no slavery, it seems sometimes man doesn't civilize if its not at someone else's cost.
However, the Bering Land Bridge is a rough theory to agree with. http://en.wikipedia.org/wiki/Bering_land_bridge
10,000 to 20,000 years ago?
Maybe those canoes are built better than we thought :)
Naw, I just think they have the years wrong. Have you ever seen a picture of the berring strait? I saw one in a history book once that showed how much water there is covering it. You don't need an ice age to freeze that, I always thought globing warming and just weather shifts account for the fact that it's now under water.