Lifespan of Games and Players?
In order to have a stable game, you need an active GM, dedicated players and clear participation rules (that need to be applied).
It takes time, for a GM, to find a group of dedicated, reliable players (they are a minority in all PbP websites, perhaps because there is no "face" involvement on line and you don't have the same commitment that you'd have face to face). Hiring on the "players needed" forum is therefore a hit and miss exercise: you need to know it, and you need to accept that most of the players openly hired there will not prove reliable (...and act on it accordingly).
Once there is a stable group of players, that know and respect each other, it is easy for any of them (player or GM) to start new games, inviting first the players that they know (many people create "white lists" and, only after they have a core group of reliable players in their games, they open the invitations to all other players on the forum or go out, hiring from time to time, hoping they will find some other reliable player to add to the group. This way, "stable" games are "born" from other stable games, reinforcing a reliable and stable group of players, that can play together, sometimes with different or changing roles, for several years.
Otherwise, it is always a struggle and you need to be really lucky.