I love you Cpt America but let me write why people don't play Trundle.
He's REALLY boring. He's one of those champions that are really strong but have those terrible not fun kits. For instance
His Q: You just use it, that's all and you get increased AD. You don't fell like you're doing anything. Sure you reduce the enemies AD but that doesn't feel as satisfying compared to a stun. Unlike Nasus's Q you can feel the "Raw power" of Nasus's Q.
His W: Again, it's one of those things you leave and ignore. Just for the buff. It's a strong steroid but man, it gets boring doing something and ignoring it.
His E: His most fun ability by far, but there's nothing interactive about it, you'll use it in one fight and it'll get noticed but it just stands there. Like the pillar it is.
His Ultimate: It doesn't feel satisfying! None of his skills feel satisfying, all you're doing is stealing stats, in a teamfight it's strong sure. But it doesn't feel* strong compared to other spells like stuns. You just use it and leave it there.
tl;dr His spells don't feel as satisfying as other spells, you just use them and leave them there. Sure his spells are really strong in my opinion, but you don't really get much gameplay from them you just get alot of stats.