I've had it explained to me as a western/eastern divide among southerners. As you head through Texas, more people think you need "all y'all" for plurals.

That's something those western southerners told me. I don't know if a linguist would agree, but that seems to be the understanding of some actual language users...

All I know is that there is a second boundary somewhere through TX, NM, and AZ, because I've never met a native Californian who would say "y'all" non ironically.