As far as I can tell, when software is open source, the barrier to people contributing is lower. And that leads to a sort of brownian jitter in the system. Once great systems, (kde in general and amarok in particular) suffer from contributions of new features, which add features but decrease reliability. Or they exist for so long that no new exciting features exist to be added, so the inevitable re-architecture death march begins (I'm looking at your kde4. You killed kde as a corporate desktop, so that you could add semantic search?).
Whatever the cause, be it the accretion of "features", or the inevitable "rinse, re-architect, repeat" cycle some project are on, it can only drive people away. But a project that is widely used collects developers, and it is difficult to say no to contributors - a common problem in volunteer projects outside software.