rom a much more distant power base across many more mountains, deserts. Also, a large population base south and West of the plateaus meant powerful, "civilized" rivals to any Northern entity (not to mention over in modern day Pakistan, separated by even more natural barriers.)
Given enough time consistently United under one political entity, one dominant culture evolved. The same thing started happening in the Roman Empire with Latin culture, but the process was interrupted.
EDIT: I worked on a pretty complex historical demography program which gave me a lot of insight into the subject.