Homogenization?

So I was looking at this map of how people pronounce things in the US, and I noticed that very few of them showed any distinct regional differences. The most striking of the few that do are what people call sweetened carbonated beverages, that thing you drink water from, and what you call Halloween eve. Bubbler people seem to reside mostly in Wisconsin, and Devil’s Night seems to be a mostly Wolverine thing. I also notice that while they ask what people call drive-through liquor stores, they don’t ask about liquor stores in general. An appellation that’s apparently unique to Michigan (I didn’t realize this until others pointed it out to me, having grown up with it) was “party store.”

Most of them just showed that the distribution of people who called them different things was pretty evenly distributed (that is if 80% called it one and 20% another, that would be as true in the deep south as in New England).

I wonder to what degree mass media and migration has been wiping out regional dialects? How different would these maps have looked a hundred years ago?