It's because American culture has dispensed with objective truth. Substantive conversations rely on objective truth in order to characterize positions. Without it, in the world of subjective truth, everyone having their own truth, you end up with Trump and the Black Lives Matter movement. Shoddy insults and incoherent yelling.
Similarly, this is why attacks on Trump's boorish behavior haven't worked. Since we also dispensed with objective morality ("You mustn't judge, dear. It's not right."), there's no standard against which one can judge Trump's behavior. He made fun of a handicapped guy? So what? Who are you to judge?
The funny thing is that this is the world the progressives hath made. This is what they've wanted since the late 1960s at least. As they own academia, the arts and most levers of the popular culture in which all of us, conservative and liberal alike, marinate, it's no wonder that a very great many of us have adopted this underlying mental model.
So Trump is a liar, a fraud and a huckster. His supporters, having dispensed with objective measures of judgment as the progressives insisted we should, don't care because they now lack the tools to define those terms in any meaningful sense.