That's an oversimplification. The reason we're "obsessing" over him is because he has never had a standout performance, he never delivers anything in the attacking third and he is prone to costly errors at the back. Furthermore he isn't showing much promise being who he is and considering his past. If he isn't delivering by the standards of the rest of the XI there is no reason to keep him there while there are two other promising left backs among our ranks. Simple as.
It's not really, people over react to every mistake he makes because he's the one still developing. There's no point giving up on him because of a few mistakes. I don't think there's another player more scrutinized in our entire team.