Trump was a highly ineffectual President, whose policies (both domestic and abroad) had no positive, lasting effect. At best. At worst, he made unmistakenly clear that the US is not a reliable partner in the future.
He didn't improve the economy, he didn't improve the situation for his citizens, he didn't strengthen their rights. He shitposted angrily at thin air, schmoosed up to Kim Jong Un with no effect whatsoever, his policies to contain chinese influence were utterly inadequate and then undermined by similar policies aimed at his strongest and most important allies, which only weakened the Western world, while china merrily expanded into Africa unchecked. Why? MUH TRADE IMBALANCE WITH EUROPE.
He had no viable political strategy, not for the US, not for US foreign policies. All he did was shitposting, ranting and behaving like an idiot. That was fun to watch for us outsiders, but it left the USA in a weaker position than it was before. America First has a nice ring to it for your yanks, but the things he did actually damaged the reputation and reliability of the US in the eyes of many allies. He wanted USA to be the unchallenged number 1 without footing the bill that this status necessitates.
Biden, as I said, is boring but reliable. GOPists going into full tardrage mode about how they can't fathom that Biden won don't understand that he won on three main pillars:
1) He's boring. Being boring is better than being annoying.
2) He's reliable. You know what you get with this guy, for better or for worse. But at least, his politics can be assumed to be stable.
3) He's not Trump. Seriously, a shitton of people were sick of Trump cause he spent more time shitposting or golfing at Mar-A-Lago than improving the lives of his citizens.
Easy as that.