Basically the reasons that I didn't vote for Trump despite voting almost always voting Republican. He's not a white nationalist but I was put off by the fact that he was responsible for the alt-right going from irrelevant corners of the internet to displaying their hatred in the open.
White nationalists have existed out in the open for decades. You were a lot more likely to see a Klan rally or a skinhead concert in the 80s and 90s than today. Ever heard of the Greensboro massacre? Unlike Charlottesville, that didn't get national attention because it didn't fit a media narrative.
If the alt-right has grown in the past few years it has far more to do with the endless media hype and hysteria surrounding it than with anything Republicans have done to supposedly "mainstream" its views. Liberal Democrats have literally adjusted their entire worldview to be centered on opposition to this bogeyman, and being everything the dreaded "alt right" is against.