It's quite easy to name some common reasons why online collaborative projects fall apart and are abandoned: Loss of motivation and commitment, poor management, major disagreements, and plain lack of time due to real life constraints.
The more ambitious a collaborative volunteer internet project is, the more specialized people, time and coordination it requires, the less personnel turnover it can tolerate -- and the more likely it is to be abandoned. And strategy games like 0 A.D. are very ambitious. They involve a very wide variety of disciplines - lots of programming (low-level coding, high-level architecture, networking, scripting, UI, 3D graphics, etc), and art (texturing, modelling, animating), and audio, and gameplay design, etc. So you'll get stuck if you don't have people specializing in all those areas almost simultaneously to get a well-rounded game, with both a good engine and pleasing artwork. Few other types of application need such a wide range of expertise. Thankfully, we have had quite little turnover, but over a decade, contributors do finish school, get married, have children, and generally have less time to contribute to 0 A.D.
Read the whole thing, and scroll down if your modern Greek is a little rusty. The English original follows the translation.