Because the concept of headcanon is obnoxious and uninteresting to most people. If I wanted to read your interpretation of shit I'd read your fanfic. It's the insinuation that your stupid little ideas are in anyway as important, exciting, well thought out, nuanced, measured, or in anyway comparable to the source work. It's borderline arrogant to throw your headcanon around unprompted.

I fucking hate "headcanon" and every one of you self aggrandizing little shits who seem to fall out of every hole in the wall anytime I want to discuss popular fiction. "Well I'm my head canon"...NO ONE FUCKING ASKED. IM RRADINT SPIDER-MAN. ARE YOU THE FUCKINY AUTHOR? DID I SPEND MONEY BUYING ON YOUR PATREON OR BUY YOUR BOOK? NO? THEN WHY DO YOU THINK I GIVE HALF A FUCK ABOUT YOUR HALF BAKED , GRADE SCHOOL TIER PROSE, FILLED WITH CLICHES AND FANSERVICE, SO FILTERED THOIGH YOUR OWN PROCLIVITIES THAT IS NIGH UNRECOGNIZABLE AS THE SOURCE MATERIAL?.