No doubt at least one of the reasons Cruz has announced he won't attack his opponents and has been very gracious when candidates have dropped out. Attacking those who are supposed to be long-term allies is never a winning move.
Cruz never resorted to attacks when he ran for Senate either. He is too principled to sling mud. He will draw contrasts between himself and others on policy, which is the whole point of campaigning and making his case to the voters. I think it works to his advantage.