fanworks/doujin culture probably.
ive only played one of the games, but i was jerking off to touhou porn long before i actually touched touhou

the pr0n itself is better than the games IMHO

Just saying "fanworks" is fucking stupid if you're not looking at why people made Touhou fanworks in the first place. Why wasn't the big 2000s doujin craze, like, Seihou, or any other doujin game of the period?

I think it a lot of it has to do with the way in which ZUN's stories basically run on the level of pseudofolklore, like Yuyuko stealing Spring from Gensokyo, or Suika punching the moon, told in this limited way where you only get glimpses of dialogue between characters which you repeat over and over and as you learn the game and each boss's pattern. Which itself is a sort of process of understanding the nature of their character through repetition. Point being, 1CC'ing any of the mainline Touhou games is like playing an arcade game recreation of being told a folktale over and over again as a child, which naturally leads to strong impressions of the boss characters while leaving the world and logic of Gensokyo vague and magical enough to offer itself to fan interpretation.

I think that sounds fancy and all, but the real truth is that touhou just lends itself well to fans doing stuff with it. not only are the characters all cute girls, which was and still is a big plus among male audiences, but also the character designs were good and so was the music, despite zun still being a mediocre artist and a fan of shitty soundfonts. in fact, his lack of polish was part of the appeal, because it meant that every character and every song was a charming but crude basis that was easy to improve upon by any and every would-be artist and musician out there. so when you combine these two factors with the fact that the internet was new and people loved doing things just for the fun of it, you get a robust internet subculture that feeds upon itself until it grows and grows to take over many otaku-adjacent communities.

the whole mythological and folklore undercurrents behind everything is pretty cool, but your average fan doesn't really care about anything but cute artwork and music and memes, and these surface-level types are the ones who make up the vast majority of these fandoms. anyway that's my theory as to how touhou exploded in a completely grassroots way that any gacha would kill to have. even now, touhou subculture is vast and deep, which is pretty impressive 20 years later. sasuga mina-san