The reason why Trundle wasn't popular during the tank meta, despite being an "anti-tank" champion
It is because although he is an anti-tank, he has 3 problems.
First, Trundle is weak to CC. I know, almost every champion in the game that is not named Irelia, Olaf, Mundo, or is a full tank, is weak to CC. But the problem is that some champions are weaker than others. And Trundle is one of them. With a meta where most team have retarded amounts of crowd control, with the top laner having CC, the mid laner having CC, the support having CC, the jungler having CC... Trundle simply couldn't keep up, because he was prone to being CC-locked to death, he doesn't have the mobility or even CC necessary to overcome that. Even though the meta had lots of tanks, those same tanks actually provided the CC necessary to keep Trundle from destroying your team.
Second, Trundle is technically good against tanks; but he generally fares better when there is only one tank on the enemy team. He is a duelist after all, also his ult is single-target. When there is more than one tank on the enemy team, Trundle doesn't tend to fare as well because there will still be one tank unnafected by him. Also, as I explained, Trundle is weak to CC. The more CC on a team, the better, and the more tanks on a team, the most CC they will tend to have.
Third, Trundle is easy to kite. Even with his pillar and his frozen domain, Trundle has a lot of difficulty trying to close the gap against many ADCs, and this is even more difficult when the tanks are here to protect them. This is also one of the reasons why he is poor at teamfights. He is one of the best duelists, but that's simply not what you want to use on this meta. save for a few exceptions, such as Irelia. Even though Irelia doesn't have the same dueling potential and tank-busting potential as Trundle, she is far less concerned about CC due to her passive. She is harder to kite too.
Back when Trundle was popular in season 4, it was because it was the "bruiser" meta - Many champions were tanky, but they didn't had the CC needed to stop Trundle. Such champions were Renekton, Mundo, and Shyvana. Renekton has only a simple stun that requires him to be in melee range, and the others don't even have a hard CC to begin with. Also, this season was far more focused on duelists and 1x1 situations, which is where Trundle excels. But on season 5, teamfighting champions are more popular; Trundle fares poorly on those situations.
I hope that people that ask why Trundle wasn't popular now understand the problem with him.
needs at least one damage item
which sets back his efficiency as a tank lategame if he's not ahead.
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