Back in 2016, after Trump became President-elect, and indeed during the 4 years of his presidency, many people (I am assuming the Liberal base) refused to accept his election, the cry being "Not My President!".
Although Trump won the Electoral college votes, he lost the popular vote, a situation that has only happened once in modern times, with George W. Bush back in 2000, and this sat very uneasily with many people.
However, during Trump's presidency, the war cry of "Not My President!" became even more vocalised and widespread, due in part to his misogyny, numerous scandals (both sexual and financial), his attempted undermining of democracy, his friendships with some of the world's worst dictators, his handling of the Pandemic, his abrasive personality, his misleading, demeaning tweets, and of course, his lies.
And oh, how he lied - over 20k of them, according to Fact Checker.
Now if Biden becomes President elect, he will have won both the Electoral college vote and also the popular vote, but already some republicans have jumped on the "Not My President!" bandwagon.
I will go out on a limb and posit a theory as to why this is happening:
"Mail ballots, they cheat!" A conspiracy theory Trump has been spreading for months, encouraging his voter base to believe these postal votes are illegal, and the reason he lost the election. Therefore his base will never/find it very difficult to accept a president who won by such "fraudulent" means.