Every opportunity to withdraw occurred before Harris was the opponent. He didn’t because he regards himself as untouchable. Something his “base” has been indoctrinated to believe and the Supreme Court has helped establish.
What happened on Nov 5th will be analysed for years to come. So I am not yet ready to give a definitive answer. Was it turnout? It seems to me that it is more likely that, as usual in politics, the current administration gets blamed for things that were out of their control, and no credit for those that were. The Biden administration took over the mess left from Trump’s handling of the pandemic and then the materials shortages and oil price hikes caused by Putin to deliver the best recovery in the west. Of course the introverted US consumer only sees their prices increasing and, encouraged by Trump, blamed Biden. Combine that with Trump’s capacity to convince the gullible that his lies are the truth and I don’t think it had much to do with Harris herself. She seems an honest person with a good heart and a genuine respect for democracy.
We don’t though really know why and right now it doesn’t matter. It is, so now we must cope, plan our own response and for the Americans to redouble their efforts to make sure the POTUS in 2029 is actually fit for purpose.
I hear that the response in Australia seems to include them deciding to distance themselves from the USA and draw closer to China. We too must find more reliable friends, at least for the next 4 years.