As I see it, Trump could not lose by letting Cruz have his chance to speak. Either Cruz would endorse (good) or he would commit political suicide (also good).
I suspect it was the seasoned political operative Paul Manafort who made these options clear because when you look at it as he would, either way it helps to unify the party. Having chosen the low-road, Cruz has less support today than he did yesterday, more of his supporters have come to see him as the arrogant, petty rat that he is, and that means they are more open to supporting Trump themselves.