Well, he would if the only choice were between Trump and Cruz. His rationale for his choice is revealing:
“I think I would choose Trump,” the liberal former president said to the House of Lords, when asked about the U.S. presidential race, “which may surprise some of you, but the reason is Trump has proven already that he’s completely malleable. I don’t think he has any fixed opinions that he would really go to the White House and fight for.”
Go here to read the rest. In other words, Trump is a complete fake saying whatever is necessary to get elected, while Cruz is a man of conviction who says what he believes and who will fight for what he believes. The view of Carter is shared I think by Republican elites who can tolerate Trump as standard bearer, a man who is not a Republican or a conservative, while gagging at Cruz who is both.