I think the reason no one plays Allistar is that he has nothing to offer after mid-game. He becomes useless after around the 15 or 20 minute mark. Here is what he has to offer:
1. A headbutt that does negligible damage... Niche use, and a very long cooldown!
2. An AOE toss that is semi-good, but has negligible damage and again, has a very long cooldown.
3. A heal that is practically useless endgame due to the high damage output being tossed around everywhere.
4. And his ult, which makes him... Tank better for a while. Too bad he is good for nothing BUT absorbing damage. There are other tanks who can do the same thing and offer useful abilities to boot.
Allistar is a cool champ, but has nothing to distinguish him as useful. I think if his abilities were buffed a lot, he would see more play and be an actual help in team fights. I know his intended role is a pusher, but after mid-game, like I said, the amount of DPS in the game makes him useless.
My 2 cents.