I think the problem is probably a combination of lack of motivation and barriers to contribution.
On the one hand, most projects are not very well documented, advertised, supported by tests, or have communities ready to help people get started in development. So the difficulty of getting started helping is probably high.
On the other hand, most projects are the unique vision of a single developer. Everyone else probably has different ideas, and other projects they'd rather work on. It takes a certain kind of charisma, influence, productivity, etc. to get past the point where people feel like contributing to _your_ project is worth _their_ time. It would be interesting to figure out exactly what it takes to make that happen, so I could use that power for my own nefarious ends...
