That is because he is too busy poisoning the well of Cruz supporters. Believe it or not, he will need them to win in November.
I think it’s the other way around. Cruz super pacs have been hitting every state with non stop anti Trump ads. More like cruz poisoning the well. I’m at a point where I’d rather see trump bring Rubio on. He was forced to get out while Kasich stayed in. A trump-Rubio ticket would screw them all.