I would say the US got Trump because the alternative, of more of the same centre-right politics from the Democrats, was not appealing. And it will keep happening as long as the only options are "slightly less right wing" and "right wing".
I've never liked the "right wing" vs "left wing" narrative, and find it extremely unhelpful. As far as I (and Goebbels and Stalin) can see, the far left and far right are indistinguishable. To me a better distinction is between those that believe in a self-supporting inclusive society, and those that don't.
JMS nailed that (and too many other things) in Babylon 5, with the fundamental differences between the Shadows and the Vorlons. And their fundamental similarities, too. Before revealing their philosophies, JMS reckoned that about 1/3 of the audience would support the Shadows' philosophy.
Horseshoe theory is
mostly bollocks. You might have a point when you're talking about extremists, like eco-terrorists, but in general it's not a useful view.
Left wing parties are generally more socialist, pro-environment, pro-workers rights.
Right wing parties are generally more pro-capital.
That's a useful distinction and in that sense both mainstream parties in the USA are right wing, just to slightly different degrees. It is always amusing to me when people say the Democrats are left wing. They are not even close.
The biggest issue with the true left wing parties is they have been taken over by, for lack of a better word, morons. That promote things like anti-nuclear power, unlimited immigration and never putting women in prison for instance (like our UK green party).
It would be nice to have a "sensible" left wing party again, one that supported workers rights, was pro-environment, recognised that unfettered immigration is bad, but wasn't afraid to go toe-to-toe with big corporations that exploit people and our environment, and wouldn't roll over the moment Russia said something mean about us. A bit like some of the Danish left wing parties for instance.
It seems that neither mainstream party in the UK is willing to do that. And I think that's where the reactionary far right votes come from here. For the US I think something similar is true. Lost voters for whom the far right says what they want to hear -- they won't actually fix anything, but they're saying all the right things to them.