Here is a good overview:

http://opinionator.blogs.nytimes.com/2012/02/19/bridging-the-analytic-continental-divide/?_php=true&_type=blogs&_r=0

I would say that there are two kinds of historical factors involved, internal philosophical traditions and external historical events. The two are entangled of course.

Partly it's a division of labor that has fallen along linguistic/geographical lines.

Certainly without WWII the history of 20th cent philosophy would have looked very different. If Wittgenstein hadn't moved to England and Carnap to the US things would be quite different.