My hunch is that there are several factors that make outside contributions easier to leverage or less so. One of course is the maintainer's ability to mentor n

My hunch is that there are several factors that make outside contributions easier to leverage or less so. One of course is the maintainer's ability to mentor new contributors into becoming co-maintainers - writing guides etc. Another would be a project's scope. If it's really small, then people will step on each other's toes. Yet another would be "fun-ness" - how intrinsically rewarding it is to work on the project. It would be alot harder to get people to stick around if they find the work tedious.