Spite, and also in case 'something' happens
I think you're both right, Nathaniel and Kaleigh, that there's no way Haley can win. She might just be staying in to prove a point and rally the non-Trump base to her side, to take a stand she can point to as prescient if Trump loses. But I think the big issue that she and most other Republicans have been dancing around is the four court cases around the country Trump is facing, his age and a range of any number of unexpected things that could happen between now and November. I think Haley might stay in the race in case something like a criminal conviction, a kind of deus ex machina for the primary that changes the story, without her having to really go after and fight Trump. The truth is he has a lot of potential weaknesses as a potential candidate, no matter how slim the potential for him to be actually sent to prison before November is. Maybe she's trying to position herself as the natural alternative should one of those things happen.
—Monica Potts, 538