The same reason why people in YOUR party fly communist flags even though communists have historically been among the worst mass murderers and torturous psychopaths. And those very communists voted for Biden so I guess Biden and every Democrat is a communist too right? (No of course not, and those who believe so are idiots.)
Another thing worth noting is that the Republicans have never been the party promoting anti whiteness that you see from Democrats today. Neither the Democrats nor Republicans today are the party of white supremacy, but since the Democrats are taking on anti white hate of course KKK aligns with the only other party. Even still, remember 2016 when David Duke endorsed Trump and Trump disavowed it right away? In fact the media asked him about it many many times hoping to get a different answer but he stood firm that he rejected the support. The first state to make Juneteenth a holiday was Texas and last year Trump made it a national holiday, Trump made a platinum plan for black America that helped achieve the highest black and latino income ever, he pledged 500 Billion dollars to help lift the black community, and Trump even officially labeled the KKK as domestic terrorists, hardly the work of white supremacists.
As for Charlottesville which you brought up, Trump condemned that too: "Racism is evil. And those who cause violence in its name are criminals and thugs. Including the KKK, neo Nazis, white supremacists, and other hate groups that are repugnant to everything we hold dear."
During Trump's 2016 campaign there was a KKK newspaper that spoke somewhat approvingly of Trump and leftists thought that was checkmate to prove he's tied with KKK until the Trump campaign rejected the praise, called it repulsive, and the writer of the article clarified that it's not a formal endorsement and he still disagrees with Trump on various issues. According to the KKK writer, he likes that Republicans have nationalism, (whereas Democrats are pushing anti American messages.)
The last major "ah ha!" moment was when there was an image of KKK members holding a Trump Pence flag, but it turned out to be photoshopped.