BTTT
The reason may also be less selfish. Cruz may thing that a President Trump, even if he's able to pull off a victory, will devastate the conservative movement as it's defacto standard bearer.
If the GOP and conservatives become the party of more abortions, tax the wealthy, liberal Supreme Court Justices (well, even more so), "sensible" gun control, any-sexual bathrooms, and on and on, what will be left of it?
Nothing.
Cruz is this country's design margin back to true conservative principles.