I left the south and never looked back, too. (Not gay, but I am non-conformist, which didn't play well in an evangelical town with more cows than people.)
In talking to people who got out vs. some who didn't, there seem to be three main, interrelated things that hold people local: money, class and fear.
- Obviously money makes moving to the Big City much easier, and more of it is always better. But below some point, it becomes incredibly hard to make work - if your parents are in the bottom half of the income gradient, first, last + deposit on a San Francisco apartment and a couple $k on moving and move-in is a huge expense.
- Class matters a lot, in that it colors how distant, urban places are perceived. Some members of my family have an almost cartoonishly apocalyptic fantasy vision about what cities are like, and the fact that nobody's sucked the marrow from my bones in 30 years of urban living will never change that.
It also effects the likelihood of knowing people who did move away. If nobody you personally know has done it, it really does become much harder to do for multiple reasons, both psychological and concrete.
- Finally, fear of "not making it", of something Bad happening, and of making a costly choice that you regret, of having to "slink back home" for whatever reason really weigh on people. Of course if you don't know anyone who's ever lived in a major metro, and if the cost of trying is big enough, that fear can massively amplify.
I moved to SF almost 30 years ago. It was a leap of faith - I landed with enough money for food for about two weeks and slept on a friend's floor for a few months while I worked shit jobs to get established. That path is harder now - I wouldn't now be able to get an apartment here now washing dishes and serving drinks. So if anything, I suspect the above is more salient now than it was a generation ago.