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