I don't think it's just FPTP, given that other countries that have it are nowhere nearly as polarized.
I think it's actually the combination of FPTP and open (or at least broadly accessible) primaries. In Canada and UK, parties generally have much more control over their primaries, and party elites generally try to exercise that power to ensure that candidates don't upset the existing arrangements too much. Not even necessarily as a deliberate strategy, but when you have to work your way through the "smoke-filled rooms" as a candidate, that filters out the purists and favors those willing to compromise. US was also like that for a long time, and notably we didn't have this degree of polarization then.
But once primaries are wide open and the party no longer controls the candidates, it seems inevitable that more extreme candidates will win. They appeal to the more ideologically motivated voters who are generally more likely to show up and vote (especially so in the primaries), and so any candidate who wants to win there has to appeal to them first.
Cycles, yes, but what we have right now is more of a spiral - there's clearly some kind pf a positive feedback loop in the system that consistently drags parties apart, and that wasn't there before.
I think it started somewhere during the Reagan admin, going very slowly but steadily at first, but the wider the gap between the parties is, the faster the process goes, so it became really visible somewhere around Obama. The relevant rule changes wrt primaries and conventions date back to early 70s, though.
I think it's actually the combination of FPTP and open (or at least broadly accessible) primaries. In Canada and UK, parties generally have much more control over their primaries, and party elites generally try to exercise that power to ensure that candidates don't upset the existing arrangements too much. Not even necessarily as a deliberate strategy, but when you have to work your way through the "smoke-filled rooms" as a candidate, that filters out the purists and favors those willing to compromise. US was also like that for a long time, and notably we didn't have this degree of polarization then.
But once primaries are wide open and the party no longer controls the candidates, it seems inevitable that more extreme candidates will win. They appeal to the more ideologically motivated voters who are generally more likely to show up and vote (especially so in the primaries), and so any candidate who wants to win there has to appeal to them first.