Because W. thinks that Cruz is sucking-up to Trump. Given the polls, I can see why.
Even before Trump became this popular, and throughout the campaign, Cruz and Trump have not attacked each other. I don’t see that non-aggression as “sucking up”. I do see portraying it that way as another attempt to attack Cruz, whom W made no bones about not liking. Cruz is the threat to the GOPee. Cruz has been a thorn in their side for a long time now. Trump, not so much, despite lately attacking all the right people. Even Trump’s 9/11 attacks on the W administration do not gain traction because they are readily dismissed by the people who just tune that out and so they are no threat to the establishment.