Boulder has (or maybe more correctly, had) that same liberal college town vibe that places like Ithaca and Burlington are known for. It is nestled directly against the foothills of the Rockies which provides great recreation options with hiking trails literally starting in town (Denver is ten or so miles east of the foothills). Boulder is home to a pretty booming tech scene, so there are some very high paying jobs to be had, too.
As I understand it, the zoning regulations designed to keep the "small town" feel prevent much development. These zoning ordinances paired with Boulder being a highly desirable place to live mean that demand far outpaces supply, and home prices shoot up.
The main explanation is building restrictions. There’s both a greenbelt and height limits.
It’s also an extraordinarily beautiful place.