Had he honored his oath of office and made his Administration about the American people vs his own corrupt and narcissistic needs, he would have been supported. But, he chose to govern as he ran — divisive and dishonest. Most Americans are unwilling to embrace deplorable conduct.
Trump’s presidency was further tainted because of the Russian interference in 2016. He both obstructed the inquiry and denied that Russian interference ever occurred. Hence, Trump made it impossible for most Americans to trust him.
However, no one seriously challenged the legitimacy of the 2016 vote count. Hillary conceded. President Obama fully cooperated with the transition.