This is the crux of the matter. There is a lot of demand for less humid, mountainous regions with lots of outdoor recreation nearby. And it’s easier than ever to scope (due to internet) out the nice locations from wherever you are in the world.
So people are taking advantage (or already have) of an arbitrage opportunity to go where they want to that they previously may not have known about, or been able to go. The population of the world also exploded in the last few decades.
There's no shortage of small towns in the US. If I threw a dart at a map of the US, I'd probably hit near one. (OK, a lot of the West is federal land but there are a lot of them.) There are a lot fewer that:
- Are in a climate you prefer
- Have recreational options you like
- Are perhaps gentrified to the degree they have food/cultural options you want
- Are perhaps at least reasonably accessible to major city/airport
- Aren't too susceptible to natural disasters
- etc.