I find it hard to believe he is not good enough to compete in Serie B, he should be 3rd choice striker easily. The only explanation IMO is that simply DD does not rate him. All coaches have their own preferences and their own favourite types of players, Bojinov is simply not in DD's book.
Kind of like Capello and DP, coaches are like that. Remember Capello used to play that idiot Delvecchio in Roma who couldn't score shit over Montella just because the latter is short, even though he was/is a 10 time better striker.
Coaches have their preferences and that's the bottom line IMO, tough luck for Bojinov and it will be for us in the long run.