Hello.
Remember when Amumu used to be one of the best junglers out there? You propably dont, thats how long ago that was. Here is why hes not the king any more:
-neutral minions no longer have negative magic resist (this makes his jungle faster)
-small camps have less health since jungle remake, that does not affect amumu too much since he is mostly using % dmg, but it made all other junglers a lot faster and thus more vaiable
-hes early game ganks were naver that good but it didnt matter since most other junglers had weak early game ganks too, this is no longer the case, now every popular jungler has strong pre lv6 ganks
-hes ultimate used to be game breaking when crowd controll effects were not that common, mercury threads not that popular and there werent any champions with cc reduction. This is no longer the case: almost every champion now has at least 1 crowd contoll ability (many of them have more), bruisers and junglers AND supports almost always buy mercury threads (APs and ADs get those too but not that often) and not only champions with built in tenacity exist, cc reduction can be picked by anyone from masteries tree.
-hes initiation through bandage toss ---> ultimate used to be feared enough to make ppl burn flashes and ghosts, that is no longer the case since more champions have blink abilities and each team usualy has at least one shurelias (sometimes even 3 of those)