There is a GitHub issue that details what's blocking stabilization for a each feature. I've read a few recently and noticed some patterns:

1. A high bar for quality in std

2. Dependencies on other unstable features

3. Known bugs

4. Conflicts with other unstable features

It seems anything that affects trait solving is very complicated and is more likely to have bugs or combine non-trivially with other trait-solving features.

I think there is also some sampling bias. Tons of features get stabilized, but you are much more likely to notice a nightly feature that is unstable for a long time and complex enough to be excited about.