Because he's pathetic. His routes suck, his instincts suck, his hands suck. He beat another receiver for a jump ball. Otherwise he's a definition of a coach killer. Just take his picture in shorts from a non-contact mini camp and that's the best Derrius Thompson you'll ever see.
Don't put too much emphasis on those meaningless games at the end of last season. I see way too many posters on this board and elsewhere doing that. Our offensive line did not signficantly improve. Our offense did not suddenly find itself due to Wanny's leash being broken. We had 1 win and other teams naturally took us for granted to some degree. The talent level in the NFL is so remarkably even it's sheer regression to the mean when a great team loses a couple late in the year or a terrible team wins a couple.
Derrius Thompson has already proven himself, a certified mediocrity or worse. If that's the level we aspire to, by all means keep him on the roster and on the field. I'm confident Nick Saban has already seen through the myth of Derrius Thompson.