He listens to Mitch McConnell.
Indeed. And, the reason is, when he wins the upcoming election but, loses a seat or two in the Senate his agenda is screwed. His second term will be at the mercy of Romney, Collins, and Murkowski.
We cannot lose a single Senate seat. Trump is aware of this so, he will listen to The Turtle, VP Pence, and his reelection team and endorse whomever he believes has the best chance to beat the democrat opponent.