I don't mean to beat a dead Colt into the ground, but the difference is that everyone else you mention (Luck, Hilton, Brown) have all shown flashes of something this season. T-Rich OTOH has basically done nothing impressive over the course of the entire season. Even when the team was playing well, we still didn't see anything from T-Rich. That's the difference between T-Rich and the other players you mentioned. Even our blind squirrel offensive line has found a couple of nuts and created holes, yet T-Rich stutters and can not hit them. The guy just isn't fast. If T-Rich had been able to hit a few more holes over the course of the season and show that he could be a dynamic runner if something clears, then people would have backed off. But he's just never shown anything.
Yes, Brown only has 378 yards....on just 71 attempts. T-Rich OTOH has just 306 yards on 108 attempts.
Besides, there has been plenty of criticism for the rest of the team. Everyone recognizes that the offensive line sucks. Luck has been criticized for making some bad decisions with the ball and not recognizing when to take a sack. I've criticized T.Y. for not being physical enough on contested balls. I know that he's not a big guy, but I still think that he could do a better job of this. Pep is criticized constantly around here. Pagano has been criticized for not having the team prepared. The defense has been criticized for playing bad prior to that last game. No one is singling T-Rich out, but the price we paid for him is a first rounder which has not been used yet, so yes it stings when you sit back and think what we could have done with the pick.