One reason is that it's incredibly difficult to punish him in the jungle before teamfights. The combination of his ult and W (and sometimes Q if he uses a jungle camp) basically makes it impossible to catch him out.
His Q also takes slightly less risk than J4's and Vi's basic mobility spells, since he can choose to go in, and has slightly more cc. Of course, VI (and maybe J4?) deals more damage overall. He just feels frustrating to play against (tho some people feel the same way about the other two bruisers I mentioned).
I don't think he's overpowered in the slightest, it's just not fun to see him too often since he's often the #1 picked jungler every season, lower win rate regardless.