They leave because Allegiance is so very difficult to learn. Simple as that. The pain of the learning curve outweighs the pleasure of the game. The newbie's experience is not knowing what's going on, not knowing what needs done, and desperately searching for some way to contribute to the team without screwing it all up. And that's if the newbie is a smart one. The problem is magnified when the only available games are very small, so a newbie mistake hurts the team much worse than if it happened in a large game.
This is the most difficult game I have personally ever attempted to learn. I might not have stuck with it this far if I hadn't briefly looked at Allegiance a few years ago, and saw how much fun this can actually be. (That first time around, I left because I was unfamiliar with how very rude people can be in online PvP. Since then my skin has thickened quite a bit.)
My few videos successfully got several people to try Allegiance, but to my knowledge not one actually stayed with the game. I'm very disappointed about that.
To somebody brand new, the conventional advice has always been to either go drop probes everywhere or babysit the miners with a nan while the vets do everything else. But in a small game - which is very likely the only kind of game a newbie will ever see - that advice is flawed. The small game requires every player to be able to rapidly switch between multiple roles.
The learning curve is the killer. But we don't want the game to get dumbed down. What would be the point of playing then?
I remember seeing one suggestion in another thread... people were talking about a kind of "good ol' days" core, with just the original four factions, but rebalanced. I like that idea. I like the notion of some means of limiting the incredible amount of trivia we have to memorize, but without dumbing down the core skills needed to play.