Well,
My uninformed opinion is that past colonialism, immigration, combined with virulent thinly veiled racism drives the gains on the right. French citizens reported (hundreds of?) thousands of Jews that French police arrested and loaded into trains east. They don't like to talk about that, but it's true.
In my personal experience, much of mainland Europe is AT LEAST as racist as Alabama/Mississippi and much more blatant about it. I'm not saying everyone, but holy shit it's not uncommon.
I say that as a white boy who grew up in the south in the 70's. I used to think it might be just because I'm an asshole, but it's strange that I only seem to have problems when traveling with my wife (who is also from the south, isn't an asshole, but IS of Asian descent). Some of the shit that happens is mind blowing. One of the funnier ones was a concierge who, with a smirk, would only recommend Chinese restaurants and didn't know of any other food options in the city of Frankfurt.