Because he doesn't relate to the DMC story as a whole?
Because he is sort of a random input in DMC?
Because people would like a game with Dante as they expected in DMC4?
Because DMC4 doesn't relate that much at all to the DMC story but giving an extra bit of confusion?
I don't know. But those sounds like good arguments why people would not want Nero in another game. Some people would rather like Dante. Me however, like both.