Trump wants a loyal disruptor who is willing to go on the attack with nonsense. Haley has demonstrated all of those qualities, and she can mollify conservative women who feel icky voting for Trump. She is the best choice for VP out of the candidates still in the race. He's not going to pick Meatball or anyone who has directly criticized him. Every critism she's leveled at the former president has been wrapped in compliments, so he'll smugly claim victory when she joins him at the podium.
And if Trump bows out, is ruled out by the courts, or is otherwise unavailable, his supporters aren't going to back DeSantis or Christie. She wins the bigotry ladder competition with Ramaswamy, and she'll say whatever she needs to say to convince conservatives that she will be their woman in the White House. Trumpers will slide over to her camp and say "see, we're not the misogynists, you are!"