They keep making new one's because, as sales are saying, that's what the RPG community wants, hell, that's what I want, it's always been good and now since sakaguchi has left the design team he hasn't had the authority to say "NO NEW BATTLE SYSTEM! ATB ONLY!"
And since then the team's been more experimental with it.
11 was an MMO, 12 was MMO style, 13 is kind of hard to judge right now, but it looks very good, and versus apparently will have a kingdomhearts-style battle system.