I think Trump knows that folks don’t trust CRuz....for President, or as SCJ.
Which is why it is a crummy idea to run President / Vice President / Supreme Court Justice / Congress all on an up and down vote in the Election.
If Cruz were going to be SCJ, they would have to keep the lid on it until after the Election, and if things turn out well for President and Senate, then ram him through. The job is perfect for him to serve us and his country, and I believe that even though I don't like him. "Elder" suits him a lot more than "King".