I can only speak for myself, but when I moved here (pre my own startup days), it came down to:
Reasonable cost of living
This has changed in the last few years, but it’s still far cheaper to live here than in NYC or the Bay Area.
Great quality of life
It’s Colorado, what can I say? World-class skiing just an hour or two (or 8, depending on traffic) up the road, biking, hiking, fishing, etc. It’s vacationland, and now there is enough work here to support a life here full-time.
Supportive community
Compared to some of the larger areas, the CO startup community is pretty small. And it’s also pretty centralized in Denver / Boulder. There’s a little happening in other parts of the state, but once you get to know people on the Front Range you’ve pretty much met everyone. And that’s great, because this is a very supportive community, really based on Brad Feld’s “give before you get” approach.
Experience mixed with upstarts
The first wave of tech activity in CO was in the 90s, and many of the success stories from those days have stuck around. So it’s not just wannabes out here. There are actually a number of very successful, experienced entrepreneurs and investors in the area who stay plugged in and engaged. Lots of great mentors, is my point.
Close to everything
Austin has this advantage as well, but there’s something to be said for living in a place that’s about 3 hours +/- from either coast. One-day trips to SF are possible from DIA, and the same goes for NYC.