This is the biggest problem with so-called humanist left: they ask too many questions. They are politically correct to the point of uselessness. Like most of the foundations that give grants for art or scholarships or stuff like that are run either by left of center p.c. guys or by mainstream corporate right, and their agendas, mostly aimed to preserve status quo, are already dangerously close. If you want to get anything from them you must carefully study how to present yourself as to fall right into their fantastically narrow guidelines: like you have to be a black, female eskimo from south Kamchatka who came to the U.S. before 1991, but not before 1989 and who love skunks.
That's where the radical right comes in: they offer acceptance to a broad range of people who satisfy just one single condition. So, it is no surprise that they have such a strong appeal. Only system on the left which was so marketable was Stalin's communism: raw, simple, no nuances, succesful. Of course, simplest ideas are usually ideas of hate, which is natural: when we desire something we are usually ambivalent and we have all sorts of complex thoughts analyzing pros and cons, etc. - and when we don't want it, we just don't. Since the principle of appeal is negative and exclusive, and range of audience is broad, inclusive, almost universal (given that all applicants satisfy the only principle), it assures for all sort of shady sick characters to join, which eventually brings any such radical left or right system down.
But their appearances through history are nevertheless naturally warranted by the inability of humans to build a sincerely compassionate political system: once they get tired of usual bureaucratic crap, they go for the extreme for few years, and then it' again the usual crap.
I am really curious about how would that turn out in the U.S.