Why do you think Donny lost the election .?

Take your pick... his constant lying, his racist rhetoric and policy, his open corruption, his friendship with Putin, the revolving door of staff and advisors leaving the White House, his failures surrounding COVID, the economy (even before COVID), etc.  The better question is "Why was this election even close?  Why did 75 Million people still vote for him?"

People lied about the number of votes he received.

As Stalin used to say: "It is not the votes that count but the people who count the votes."

Covid Incompetence.

The economy in the covid tank.

Lies

more lies

Incompetent administration picks for the cabinet.

Joe won more votes in more states to win both the Popular vote and the Electoral College votes.  

Trump showed what a big baby he is and always was after losing. 

Trump was lazy, and due to his unwillingness to subject his fragile ego to negativity, he refused to take any political risks--he only "campaigned" among those who were already gonna vote for him no matter what--his rallies were held only in "friendly" territory, thus wasting time and campaign dollars on people he didn't need to convince--instead of  even attempting to hold onto or attract more moderate voters.  That's why his rallies were so huge--they weren't so much campaign rallies as they were  just venues for him to strut around on stage and have his ego stroked.  So, he lost a significant proportion of those who had voted for him (or at least against Hillary) in 2016.  He lost significant support in the suburbs, among both white women and men, especially those who were educated.  They were fed up with his pissy, nasty, childish attitude, his lack of professionalism, his incompetance and his nonstop lying and bullshitting.  The icing on the cake was his dismal incompetance and dismissive apathy in handling the COVID-19 pandemic.  Finally, non-white voters turned out in higher numbers than in previous elections, and he had already spent 4 years burning his bridges with them.

So, Fatso did it to himself.  And true to form, he tried to blame others for his own abject failure--including making blatantly false and dangerously irresponsible claims about "election fraud."

Another answer states that his crimes caught up with him.  Well...sort of, but not really--THAT can't really happen to him until he's out of office.  What DID catch up to him and bite him in his fatass was what was listed above--his laziness, his incompetance, his  narcissism, and above all his propensity for bullshit.  His business model was always built around the notion of "fake it till you make it."  That may have worked for him in his style of doing business, but it ruined him as a president.  One can get away with bullshit in business if you can freely throw money at any resulting problems, but that approach just doesn't cut it when occupying the Oval Office.

Because he got less votes than his opponent.

Because the majority of people never wanted him.

He never managed an approval rating of 50 or more percent throughout his term in office while his disapproval rate was always above 50%

The only way that he could win was to win the electoral college despite losing the popular vote. In 2016 he managed it, but not this time.

Novel COVID-plain and simple. 

Because he got Joe a lot of sympathy votes by attacking him personally instead of just his policies.

He didn’t lie enough. People wanted a liar in the WH.

Fat Donnie's crimes caught up with him.