Cruz has commented several times that he doesn't want to get into the trashing of his opponents. I think back at the debate - Paul bashed Trump and then Paul and Christie got into it. Yes Paul is entitled to voice his opinion but he did it in a way that came across as child-like; especially the roll of the eyes.
It's obvious Cruz would rather focus on the issues right now rather than waste the time and energy attacking. He's done nicely in the polls without having to do so. He also is busy with his Senate duties.