Senator Cruz, as the master debater he is, should know that you don’t do anything to give any credibility to your foe when you are clearly in the lead.
It also appears Ted forgot he’s had 25+ hours of debate with Donald, already.
No actually Trump has had 25 hours of debating bozos Jeb and Marco on the stage with him, that Trump used alternately as a punching bag and distraction to prevent having to talk substance on many issues.
Only when it got down to four was Trump forced to defend his non conservative positions at length against Cruz. And Trump fared badly and won’t debate since.
And not just credibility, but when you're ahead, you have a lot more to lose than to gain. Why risk doing something that might (somehow) change the status quo? When you're behind, you have little to nothing to lose.