Because he's a nasty spoiled little brat, and a sore loser to boot.
If Boris takes the reigns, I think I'm right in saying that he might try to renegotiate our terms, then go for a second referendum. Camoron is holding out for that, because he said he wouldn't hold a second referendum. - Mind you, with his history of lies and U-turns, a move like that wouldn't be unexpected.
To my mind, Camoron is the worst PM in living memory, and that's really saying something.
Of course it'll fit; you just need a bigger hammer.