Two party system. As many people didn't like Hillary, clearly there were a lot of people unhappy with Biden->Harris. When you don't like the current admin's direction and/or their party, there's only one other party to select. I think there were plenty of voters that truly did not believe this would be the result of that protest vote.
Prior to 2020, I usually voted for third parties so I do understand that kind of thinking. The danger Trump represented was not obvious until well after he took office; it seemed early on like congress and institutional norms would restrain him. To swing the popular vote in the 2024 election, almost all of the third party votes would have needed to go to Harris, so I don't think that's sufficient to explain it.
By the end of his first term, the danger was hard to miss, and the attempt to remain in power after losing the election should have cemented it for everyone.
I was unhappy with Biden and Harris. I voted for them in 2020 and 2024 anyway because I understood the alternative.
Protest votes are probably overstated, I think most of it comes down to people staying home. Everybody in America already knows what side they're on, and they either vote for that side or not at all. Virtually all political messaging is either trying to moralize your side or demoralize the other, to manipulate the relative ratios of who stays home on election day.