Because he doesnt have a grab and slow or never builds a frozen mallet. And he can NEVER burst squishy champs down then ult in the 3 seconds it takes for slow to GTFO.
Honestly his passive needs to be tweaked down again, his q needs to be tweaked down and his ult needs to not FULLY refresh when he kills with it. Cho'gaths doesn't and cho has a HELLA smaller range than darius does, plus all of his kit is dodgable even if he catches up to you.
But w/e, no one ever seems to agree that the bruiser genre needs a nerf *cough* garen *cough*
As for the Garen comment, let me repeat that bruisers fall off late game. Lee Sin, Garen, Darius, Wukong, and others will often be outscaled to death if they can't win early and will be of little use lategame other than as a meatshield.