Because they hate America. Next question.
Oddly, "decent" has nothing to do about it. The Leftists are died-in-the-wool Revolutionaries that believe their values are morally just.
The fervent Democrat/Communists are religious in their hatred for Republican/Constitutionalists.
Either these ideologies separate into two, new countries or there will be a bloody, retracted warfare with no country surviving.
The Democrat/Communists have undergone the Muslim framework that they are willing to die for their beliefs, while we are soporific in unwilling to kill for ours.