It's a bit naive to think that he's only not playing because he's shit. The fact is he's an unknown. The manager doesn't know him, the coaches don't, King doesn't, the players don't. At this stage a handful of training sessions where Rowett is trying to get a grip on the squad as a whole, he's not going to be focussed in on a loan striker who has also just walked through the door. Carranza was given the opportunity to disappoint at least. Richards doesn't really have time on his side to both integrate and impress.
Mukasa walked into the side and obviously looks a player, but thanks to injuries and suspensions to the usual 10's he got an opportunity that Richards might not get.