Remember in 2012 when the GOP was desperately searching for an alternative to the "moderate" Mitt Romney? Such as Jeb Bush? Or Chris Christie?* But all that was left were two "conservatives," Rick Santorum and Newt Gingrich. And remember how they couldn't "unite" to stop Romney?
This year you have an "outsider," Donald Trump, whom the party bigwigs are also desperate to stop, with the only acceptable alternative, Paul Ryan, demurring on a run, a la Jeb or Christie. So, like Santorum and Gingrich, only Ted Cruz and John Kasich are left to stop the Donald, but they can't quite get it together either.
Oh, and remember how Romney only carried 24 states for a total of 206 Electoral votes? Trump will be very hard-pressed to improve on that. (My guess is he will fall quite a bit short.)
* Two examples of what George W. Bush meant when he said, "The point is you don't get to pick the environment in which you run."