There is often a perception that a project is "done" and further contribution isn't needed. This is almost never the case. (bzip2 was given as an example, as was libexiv)
There can be a distaste for getting involved in older projects that have a lot of technical debt.
Sometimes the existing community impedes the project from evolving to follow the interests of the ecosystem of developers
Loss of inertia in a project causes the best & brightest contributors to leave for more interesting spaces, causing a brain drain
There are two types of contributors, "creators" and 'maintainers". Creators leave when projects become stable, maintainers stay.
Often a lack of awareness of what a project needs. Somebody suggested creating a "crisis" that will put focus on those needs
It can be hard to find a new leader to take over a project when the current one needs or wants to step away from it
A proposal was made to "archive" older projects, making them available for future reference or use, but otherwise taking them out of the active projects ecosystem
Suggestion to look towards students as possible new contributors, work with faculty to find ways their students can get involved in well established projects
Make sure you have community buy-in to your core values and mission.
Two questions potential contributors ask: "Are they like me?" and "Will they like me?"