Why is this evil ideology attractive to them?
To put it quite simply, because they are people who are vehemently afraid of equality and what it means for them. To expand a little, the lack of their overt supremacy is something they cannot abide by - being equal with the people they have learned to hate is tantamount to being equal with filth. Toni Morrison talked about how racists fear the absence of racism and realising what little worth they have. If the white supremacist structures that have supported them are failing, what do they have?
Earlier I mentioned the word learned, as we all know that this kind of loathing is not an inherent feature. Learned by way of family, or learned by way of a poor interpretation of the world around them, combined with anger. We all get the general idea. It's a way of comforting oneself about their place in the world - I may be (x), but at least I'm not a faggot, a tranny, the n word and so on and so forth. The same kind of thinking goes for pride - I got to where I am today because I am a superior human being. Or because they have a cognitive dissonance which allows them to take part in well-established white supremacist structures without guilt. Just playing the game or what have you.
But that's a simple elaboration on stuff we know, or at least, should know. In terms of America, this is nothing new: white supremacist and eventually neo-Nazi ideals are inextricably linked to the country. Trump isn't even the first president being alleged to have personal identification with these ideals, you have presidents with similar murky issues. But as Kanzler mentioned, what is different is that it is being legitimised at a level which has not been seen within the majority of our lifetimes. This time, white supremacy is about reclaiming a time in which they believed they were unquestionably on top. This is different to previous flare ups where white supremacy's overt goal was extending the status quo. I am of the viewpoint that white supremacy has of course never left, but the key thing to understand is that these people believed that it did, and when it disappeared, so did all their fortunes and all that was good in the world. And while successive presidencies have played to this abhorrent ideal, none have been so overt in courting the idea that through this administration, you may rise up and reclaim what has been lost to you. Not that the Trump administration is solely to blame, of course. But there is something to behold when the highest office in the land, the definition of America for many (and usually the people who hold sympathies for these beliefs in the first place), is telling you that you are legitimate. If the president and his people are sending you signals that are playing to your sympathies, it is no small wonder that these people picked them up and ran with it.