The problem is that people using Steam or similar services expect complete, functional games. 0 A.D. is not complete, nor completely functional and requires quite a bit of polish (look at all the discussion about performance issues). So bringing 0 A.D. in this alpha state to Steam would generate a lot of complaints and perhaps cast a negative picture of both this game and Steam (as a portal to good, carefree gaming). I would advice against it until the game is better able to deliver on its promise (please don't take this the wrong way, I like the game, it just needs some time). For now, it seems wiser to invest time and resources in the game itself or attracting developers, rather then growing the number of players.