Because animals normally are limited to LOCAL phenomena rather than GLOBAL ones, foundationally.
Evolution first assigns value by EACH independent animal's treatment early in life during 'windows of development'. These assign the concepts of 'good/not-good' and 'pleasure/not-pleasure', etc, from these. We learn values from (1)our self (our literal biology in context to our immediate environment, then (2)those people and things which provide these initial developments (our parents, direct guardians) then to (3) extended family and then to (4) others with SIMILAR appearances (which broadly include anything our emotions giver rise to), and...rarely, by normal evolutionary standards, beyond this.
"Family" is what some right-wingers would often use as a word to defend their focused interest because it justifies why they would believe one should CAPITALIZE on ANY behavior that gives this inner circle their advantage over others. "Race" is just an extention of this bias, as it is against other "species" altogether.
And the reason we get a rise in this is not directly from the right-wing groups because this is already default to us on a SELF-interest level. It occurs DUE to their KIND of thinking though. I think the indirect but more powerful CAUSE of fascism comes FROM within the ranks of the left who can overpower the interest of people as individuals among the whole. They are GROUP-defining right-wingers who find utility on the left by "collective fascists" that make these particular groups become the DEFINING minority rather than the individual.
So Trump's success, for instance, was as much due to how the present cult-like advocacy of the extremists who interpret an us-versus-them mentality on many "#meToo-like stances where the 'me' is defining some shallow genetic relationship as co-related to 'culture', like that one might believe that one OWNS some artistic skill due to some ancestral link, when these are ARTificial constructs.