The ONLY reason that a Bush or Rubio voter would vote for Cruz over Trump in a two-way race with no GOPe alternative is to ensure a demorat victory in November, which some of them I believe would clearly prefer.
They know what most of us know: Cruz cannot win in November.
As one FReeper said (and I wish I bookmarked it and I wish I remembered his name):
Cruz has all the electability of Goldwater and all the charm of Nixon.
We also "knew" he couldn't beat Trump in IA.
His opponent is likely a devout elderly Communist... or Bernie. ba-dum-PAH! ;)
Even Mittens would have a chance in that race. Declaring that Cruz "cannot win" I think says more about what you hope for rather than actual realistic expectations.