My theory is not that mythology kept you together, but that there was enough cultural and physical space to peacefully ignore each other if anyone so wished, simply by moving a bit further away. Communications were slow enough that they could be digested or ignored more or less at will.

The problem is that, between natural population growth and the rise of the internet, both the cultural plane and the physical one have largely run out of space. Figurative "midwest zealots" just cannot live their lives ignoring "coastern heathens" even if they'd like to, and viceversa. And so the fights have broken out and will likely continue for a long time.