Could be in part to how his kit as a stat-drainer is kind of hard to get across visually, so people don't appreciate that power.
Granted, his abilities definately show that "TRUNDLE IS BEING BUFFED" (Glowing club after Q, red-aura in Frozen Domain, bigger when he's used his ult), I think only his ultimate shows any sort of Debuff visual to it (The energy coming out of the ulted enemy and onto Trundle).
Also, I don't think his passive has any sort of visual indicator at all, so if you didn't know much about the champion, you might just think he has a above-average HP/5, and no passive.
And while I strangely find him fun to play, I can see why most people don't play him: besides his Pillar, he doesn't really have any way to out-play someone (And even then I'd hesitate to calling his pillar an 'outplay'): He pretty much just out-stats his enemies, is how I would describe it.
EDIT: And like others have mentioned, he can still be kited, since most of his damage comes from being in your face (He has his ult and BoRK, but I don't think you're often going to use those to secure a kill, but rather use it to start the fight.)