Societies degenerate because people tell themselves the lessons of history do not apply to their situation.
People think that their situation is different - which it always is, usually in some trivial way - and therefore use this insignificant difference as a pretext to waive away said lessons.
For example: Nazis referred to Jews as vermin. Hutus referred to Tutsis as cockroaches. Trump referred to immigrants as an infestation. Not exactly identical so therefore lessons of history don't apply. Or so goes the self-delusion.
Except history never repeats itself exactly. The lessons of history are intended to be broad, not narrow. They are meant to be a deterrent, not to describe an exact replication.
This is related to exceptionalism. Humans tend to view themselves as exceptionally altruistic. This permits them to dismiss any negative warnings.
But how much do we learn about historical examples in which movements with totalitarian and/or racial/nationalistic bents, etc. were thwarted by actions of civil society?
We're often presented with negative blueprints but rarely affirmative ones that can be acted upon.