Only Trump knows why he decided to give a prime time slot to Ted Cruz when he knew, based upon an advance copy of his speech, that Cruz would not endorse him. It might well be a final sign of contempt for a defeated adversary who Trump believes is now harmless. “Let him say what he wants”, Trump might have thought, “what do I care?”
As for Cruz after the personal attacks that Trump had made against his wife and father, he could not endorse Trump. He plans to run in 2020. He knows that there are two possible outcomes in the fall. If Trump loses he will not be associated with what most Republicans will then regard as a mad episode in the history of their party. If Trump wins, Cruz likely assumes that his Presidency would be a train wreck of epic proportion and that if Trump runs again he will be vulnerable in the 2020 primaries.
This morning Cruz appears to be a Judas to all the pro-Trumpers and to much of the GOP. To the vanquished Never-Trumpers he is a hero, the champion of conservatism that did not bend the knee to The Donald. Cruz is playing a long game of chess and he is probably where he precisely planned to be. Well played Mr. Cruz, well played.