From my observation, the causes tend to be:
- Maintainers spread thin, putting more of a burden on contributors
- Some projects being critical and needing more attention and some taking themselves too seriously (dealt with a project whose CI took a couple hours for ~60 jobs because they exhaustively tested every combination.
- Maintainers who haven't learned to distinguish the important from unimportant.
- Contributors who don't realize their PR is a white eleplant (burdensome gift)
- Contributors who jump in with a PR without first seeing if it aligns with the project.
Most of this happens everywhere. Probably the more unique aspects to Rust are being spread thin, precedence of high quality crates, and that Rust gives a level of control that is addictive and effects other parts of projects.