Hear me out on this one…

Trump signs the pledge. Since the RNC is a private body they can technically choose anyone they want and choose someone not Trump. Trump sues them bringing the question of the RNC/DNC into play. Trump further runs independent causing the RNC to sue him to keep him off the ballots (again also putting the RNC/DNC into question). While things work their way in the courts, Republicans have a split ticket problem.