Disclaimer: I went for a memey title, but hear me out. Also, I want to stress this is a theory, not something I think is reality...just something that could possibly be real. I don't think it's the likeliest explanation, but it's something that popped into my head. We'll see what happens if she drops out today, too.
What this comes down to is the age old question: Is an organization - in this case the GOP - more evil, or more stupid? Essentially, are they running things competently?
They don't seem to want Trump. But they know swapping him out would be a disaster. What if Nikki Haley is a sacrificial cutout to try to dampen the blow? Here's my idea: They're pushing Haley to stay in, precisely because she's hated but, if Trump gets taken out one way or another, they're not going to put her in as the candidate, even if she's still in the race...they're going to swap back to DeSantis, who recovered some of his popularity after dropping and endorsing Trump, and was also polling higher. It would make the whole process seem much more palatable to some. I personally don't think it would be enough, but it's certainly better than running Haley.
Essentially, Nikki Haley is in the race to be hated, so the GOP can stab Trump in the back and swap back to DeSantis or something, and pretend it's for the good of the people and the party.
And, since I know we probably still have some DeSantis fans, I'm not even saying he's in on it in this hypothetical situation. He could be, in one version, but it's certainly not required. Dropping out and endorsing Trump was a fairly natural move, and is in no way a condemnation on DeSantis.
No matter what happens, this election cycle is going to be a chaotic mess. The cleanest option is somehow a Trump v Biden rematch. Anything else gets even stupider, and even less representational.
I think the eGOP are happy to lose and be the minority party so they can grift their base on what they'll do if they ever get back in charge. So if Trump falls they'll pretend to care about it while they support everything the Democrats want (Ukraine etc) since they are all the same Uniparty.
Letting a Nikki artificially represent the party in that scenario is another "win" in terms of diversity if nothing else.
Both parties enjoy swapping out who is the minority party. Brings in more cash donations being second.