Because he was running against an even bigger liar who had just completely wrecked the American economy with his utter and complete ineptitude, after leaving allies in Syria to be slaughtered by Erdogan, had been impeached for attempting to sabotage another by illegally withholding congressionally allocated aid unless they announced a fake investigation into his trashy politician opponent, and frankly, and spent four years sucking up tens of millions of taxpayer dollars by overcharging the government for amenities at his own properties.
As for the votes that were legitimately counted on election night, there were dozens of hearings in as many state legislatures by election officials begging Republican lawmakers to allow mail in ballots to be counted in their states before the polls closed on election day because THEY KNEW this was going to happen BECAUSE he completely botched the pandemic response, and so a historically unprecedented number of votes would be cast by mail. Those Republican lawmakers, of course, didn't listen to the experts and so we got exactly what everyone but you guys expected.