Reddit mentions of American Voices: How Dialects Differ from Coast to Coast

>How does the extreme regionalism maintain itself?

Why shouldn't it?

People like the way they talk better than they like the way those other people talk -- in part because it's what they're most used to using with the people closest to them, and in part because it identifies them as part of their group.

A Northumbrian doesn't necessarily want to sound like a Londoner any more than a Texan wants to sound like some Yankee from New York (or any more than a New Yorker wants a drawl) despite each knowing full well what the other sounds like.

It's also not just geography; social class has a lot to do with it as well -- especially in England, but really everywhere.

>I understand that 150 years ago, people didn't get out. You hear tales of folks who would go their whole lives and only leave a 20-mile radius of their home a handful of times. I understand how being that insular breeds extreme regional dialects.

You overestimate the amount of turnover, I think. People can visit across England easily, but nobody changes how they talk on a visit. They can and occasionally do move across the country -- but then they usually start talking a lot like the people around them, especially if they're young when they move. If the population turnover in any given location is only about a percentage point a year (that's high for most regions) and it takes maybe a decade before you sound like a local after moving (that's also high, for most people), you can have regions keep their unique dialectical characters indefinitely, especially if no one has any particular interest in convergence.

The reason the US has so much less variation than the UK is that it was settled relatively recently by people from all over the UK with speakers of different dialects mixing (in somewhat different proportions) almost everywhere. But towns in the UK have been continuously inhabited by mostly the same people, those people's descendants, and proportionally small numbers of people who joined the communities from elsewhere for over a thousand years. In fact quite a bit (but by no means all) of the dialectical variation within England stems from a 200-year stretch of Viking rule over roughly half of England and not the rest. It stays, because people from Nottingham would rather sound like people from Nottingham than like people from Dover.

And if anything, dialectical variation within the US is increasing rather than flattening out. This book is the best introduction I'm aware of to American dialectology and really a pretty good introduction to linguistics in general.