Annoyingly, the main reason Parler (and similar services) became so extreme was because the mainstream communities, organizations and platforms kept censoring conservative thought and thus driving them elsewhere. Then you end up with an echo chamber formed partially around the dislike of an outgroup... Radicalization inevitably follows. If people and institutions had just been more tolerant about allowing conservativ…
I would argue the opposite: the reason those sites became so extreme is that the mainstream sites restrict so little but the very extreme, that the population looking to escape the mainstream sites is very extreme.