I think it might have something to do with Cruz’s ambitions. If he wants to sit in the chair he would accept an offer as VP. If he was willing to serve as AG then a Supreme, I believe he would become an historical figure.
Sen. Cruz would be a great candidate for any of those chairs. And knowing how the left and not only the left but his own party, hates Cruz and everything for which he stands, the man could never be elected or nominated on individual accomplishment. The two, Trump/Cruz, seem to have worked together to perfect this dance and both have the opportunity to succeed in their ambitions - MAKE AMERICA WHOLE and GREAT AGAIN!