homogeneous societies have an easier time of running collectivist programs without incurring wrath of the populace. no one minds if tax dollars go to other people just like them.
Sure, and there are also lots of other things that work well on lower, local levels, too, but that might not be appropriate for national-covers-everything-and-everyone style laws. But that's are not cut and dried, either. Some stuff works better at a national level.