Because Trump is a bully and Bush is the nerd who doesn't quite know how to stand up to him. Rubio played the part of the good-guy Saturday when he said I for one thank God it was George W. Bush in the Whitehouse on 9/11 instead of Al Gore. That was a great moment for him and may have turned things around a bit more for him. We'll see. I'm afraid that at this point, to beat Trump will require either Cruz or Rubio to drop out, that would seal Trump's fate and he knows it, its probably why he goes after Bush so much, kind of a smoke-screen, he doesn't want Cruz or Rubio to drop out, he doesn't care if Bush does.