I'd say it was the nature of US history and culture that sustains his ratings - particularly with regard to race. He's just revealing what has always been part of US culture and society. The US self-mythologises to an extent that conceals how much darkness has always been there.I’m pretty confident that the only things keeping Trump’s approval from being in the single digits are partisanship and the personality cult, both of which I expect to fade over time. I would not be surprised if in ten years he is viewed as the worst president in US history. (Lucky for you, Buchanan!)
I'm also fairly confident he'll come to be viewed as the worst US president ever, though it may take more than a decade. Even if politically-more-harmful monsters emerge in future, it's hard to imagine there'll ever be one as ludicrously and openly psychologically-dysfunctional as Trump.