Cimorene

I make several things of it:

- fandom loves to woobify characters, usually the more ridiculous the better;
- woobification of morally gray or dark characters is particularly prone to including intense manpain and abuse 'showing' in the narrative how none of it was their fault (eg Loki in MCU, Methos in Highlander);
- as OP said, fandom also loves the righteous underdog who emerges from under a hail of undeserved self-righteous abuse by what turn out to be assholes and is proven to have been better and more righteous all along (like Matilda or Jane Eyre);
- and fandom loves wish-fulfillment style The Chosen One stories where the hero who emerges from trials and hardships was destined, or else turns out to secretly be the crown prince or the most noble/magic/qualified of all, like Harry Potter or Merlin or the Princess Diaries (hence all fic about how Crowley didn't do anything wrong AND God actually secretly liked him more all along and that's why he was chosen for his special mission).

There's probably also just an element of the fact that it's easier in general, and for many people more fun, to use more tropes and cliches rather than less of them.