What would you say is the biggest factor in Donald Trump's inability to accept that he lost the election?

The fact that the voting in swing states was even closer than anticipated and tripped need for some states to do recounts and audits.  I truly do wish that people really understood how elections work.

The fact that they have not declared officially any outcomes yet?

He suffers from narcissistic personality disorder.  Here is how the Mayo Clinic describe the symptoms of this mental disorder;

Signs and symptoms of narcissistic personality disorder and the severity of symptoms vary. People with the disorder can:

Have an exaggerated sense of self-importance

Have a sense of entitlement and require constant, excessive admiration

Expect to be recognized as superior even without achievements that warrant it

Exaggerate achievements and talents

Be preoccupied with fantasies about success, power, brilliance, beauty or the perfect mate

Believe they are superior and can only associate with equally special people

Monopolize conversations and belittle or look down on people they perceive as inferior

Expect special favors and unquestioning compliance with their expectations

Take advantage of others to get what they want

Have an inability or unwillingness to recognize the needs and feelings of others

Be envious of others and believe others envy them

Behave in an arrogant or haughty manner, coming across as conceited, boastful and pretentious

Insist on having the best of everything — for instance, the best car or office

At the same time, people with narcissistic personality disorder have trouble handling anything they perceive as criticism, and they can:

Become impatient or angry when they don't receive special treatment

Have significant interpersonal problems and easily feel slighted

React with rage or contempt and try to belittle the other person to make themselves appear superior

Have difficulty regulating emotions and behavior

Experience major problems dealing with stress and adapting to change

Feel depressed and moody because they fall short of perfection

Have secret feelings of insecurity, shame, vulnerability and humiliation

He’s a baby with a fragile ego

His ego and upbringing. His dad said that if you don’t win then you’re a loser. He didn’t win so he’s a loser and that’s something he can’t accept. 

All his life, he can fix any problems with money. Poor SAT scores, get someone to take it for him. Business not doing well, declare bankruptcy and let it be the banks’ problems. He thought he could buy the election by going to court but that’s not how it works. 

The fact that millions showed up to his rallies and vocally supported him while Biden could hear crickets at his rallies.  Also the fact that it is hard to believe anyone would vote against him when he had such a strong economy, low unemployment, and put American companies first.

Biden has been in politics for 47 years and I have yet to find a democrat who can point out anything of worth that Biden has ever accomplished.  How does the majority vote for that?

Then the fact that Biden's running mate is the most radical left-wing politician out there. 

I do not see how it is possible that Trump lost either.

His terminal narcissism and utter stupidity.

The 1000's of dead people who managed to show up, and vote?

Umm the fact that sleepy Joe got more “votes”.

His ego and his narcissism