Maybe they will normalize his Ult cost when he gets a nerf. He's so abusive in solo queue. You'll never see him in pro play, but he is one of the strongest tank junglers for Bronze-Plat. His bandage toss has one of the fattest missle widths in the game, he outputs AOE %Max HP damage constantly, his Tantrum gets CD from basic attacks, and he gets physical damage reduction, his passive reduces MR when he AA's, and he has a 2 second AOE "entangle" (stun).
Mana gating is a good way to keep a champion from being overbearing. Think about how completely broken Pantheon would be if he didn't have early mana gating. Amumu is just on the opposite end of the spectrum, having lategame mana gating to keep him from being more obnoxious than he already is.
Really what it boils down to is that Amumu is an old champion, and older champions have a history of having increasing mana costs on their Ultimates with levels. The reason that some champions still haven't been normalized to more appropriate mana costs is most likely they are not in need of any buffs at the moment. In Amumu's case, he has a substantially high winrate in soloqueue (he's been 50%+ for almost 3 seasons now), that only increased when Cinderhulk was introduced.
If the new Abyssal Scepter makes it to live, he will be even more abusive. He will probably see nerfs after it is introduced and then finally his Ult might get a mana cost reduction. That is my prediction anyway. Good luck out there. Peace.