>>1881472The IP is free if you specify it's a 2hu fan project. You can do anything with it and profit and ZUN won't take a dime.
That means it had shit ton of content produced for it. And I mean a SHIT TON. Fangames, doujins, artbooks, music circles for all genres.
It's a bit old and busted since it started inclining in 2002 and peaked in 2009, but I'm not exaggerating when I say that at one point 90% of comiket was Touhou.
It was the peak of SOUL and that's why you see a lot of people obsessed with it until today and a bunch of these boomer otaku girls that look at it very fondly.