Community momentum, corporate support, and how significant the changes were between the last two big versions. That last point was a big determinant for me when I was deciding to learn react vs vue. The vue 1 -> 2 upgrade broke a ton of community packages, plus a lot of unhelpful tutorials. In the end I chose React over Vue, and only _sometimes_ regret it :)

Edit: also, active reddit community. I don't ask a lot of questions there, but it's a reasonably good metric of the health of something IMO.