they weren't? he quit in july with months of polls showing him down as much or more in all the states she lost, and he was also losing virginia and NH in some polls (where she has clung on). his approval rating has been sub-40% in all polls including exit polls after the election.
if you're comparing to 2020: covid and trump's handling of it was a big issue, raising questions about his competence, people had disliked having to hear about politics all the time, biden was the most generic politician imaginable, and would presumably fix those issues. in this election, the issues were different - no covid, it was about inflation and immigration. people want to see immigrants in camps, and they want cheaper burgers. they remembered the low inflation and tough rhetoric and actions of trump against migrants, and punished harris (and the biden admin she was part of) for the opposite.