It also would have been a poor career move. I wouldn't count the Tories out yet, but there's at least a 50/50 chance they'll lose the next election (and that's being wildly optimistic) and if they do, Harper is gone and MacKay is the frontrunner to replace him, with only one real challenger. And once he does, he'll be up against a tired leader twice his age who voters have only been re-electing due to an absence of alternatives. When Brian Tobin and Jean Charest answered to call to come home, they were nowhere near becoming Prime Minister - MacKay is.
Of course, was it ever really in doubt? After all, what kind of person would bolt from a new party so soon after they helped to create it?