His clear time is extremely mediocre compared to most junglers these days. He's a great duelist if their jungler is just going to stick around and fight, but they won't. His pillar is the only real tool he has to trap enemies and to make ganks happen, and it's really nothing more than a slow that looks like a penis. Almost everyone takes flash and a good number of high-tier champions have some kind of escape already, making it far from great. This applies to a lot of junglers, but Trundle doesn't have a great clear speed to make up for it. His animations are abysmal which makes chasing anyone a chore, even with his movement speed buff. You won't ever solo dragon at level 6, because they will have dragon warded. I disagree about his damage being noticeable, it's about as low as it gets.
Trundle is a champion designed for a game which no longer exists. In the old jungle his insane sustain and great security in his own and the other team's jungle was amazing, in the new jungle he just doesn't work. Modern junglers must have either great ganks or a quick (aoe) clear, and he has neither. It's the same reason Nunu stopped being viable (for a while). Power creep and jungle changes.