When Cruz (and Rubio, remember?) decided to go on the attack against Trump, they could not pull it off without seeming mean and snidely (Cruz), or vaguely hysterical (Rubio).
Trump has a way of attacking without seeming mean and vengeful. His personality makes it seem a little funny.
When Trump attacks, we nod and chuckle.
When Cruz (and Rubio) attack, we go "ewwww!"
It's a personality thing. Not a lot they can do about it...except to learn what works for themselves, and learn never copy someone else—especially someone as outrageously unique as Donald Trump!
That is when the Cruz and Rubio trains derailed. When they tried to out-Trump Trump. It was not only impossible, it was just silly.