He has some awesome support abilities, but the main reason people don't play him is that he doesn't have much to make up for his lack of range. His stun is useful, but incredibly short duration. Any team that has a moderate amount of cc can avoid him altogether. I play him in ARAM though and he's a lot more useful when you have the whole team with you.
Any other support that has CC or long range can abuse his limited range far too easily, but in ARAM he has the snowball to instant dive and stun or even Ult the whole team.