Not much to wonder. If you say something long enough and loud enough, it becomes the truth. Irrespective of facts.
Goebbels.
Or the Cruz campaign has been caught lying and Cruz has apologized for it. More than once. I would guess that is the reason more than what you stated. The problem with Cruz setting himself up as the morally superior candidate is that he is then expected to run a very clean campaign, which many people perceive he has not.