>>921It's a little bit of everything. Aside from the gameplay, music, art (despite people poking fun at it), cute girls, and being in the right place at the right time on 2000s internet, I think it grew because the world is as open-ended as it is. There isn't a cool plot or any defining climax, and at the end of every incident everything goes back to the status quo, but that's why it's so appealing. Gensokyo is a world, not like those made by modern worldbuilders who over-detail everything from the start without any purpose and think they're going to be the next Tolkien, but like Wind in the Willows: a place you can daydream of, between old and new, between fantasy and reality, and as whimsical as can be. What you see in the games and writings is just a portrait, a brief snapshot of that world, but the rest is on the other side. Artists and writers and even completely unskilled fans latched onto that so they could explore it.
Seeing picrel this morning helped me put it into words. They're on some whimsical errand in their old-fashioned motor car, and you can imagine what happens before, during, and after. At the end, everything turns out fine. I daydream a lot, so this is all a personal view, but I think Touhou is something stronger than fantasy: it's a fairy tale. That's what makes it different and special.
If you want to see a small part of how it grew, here's an oekaki board that goes back to before PCB was released. It's fun seeing people make art for characters as they come out.
https://coolier.net/oekaki/191.htmTo keep it on-topic after all that, I'm working on my first lunatic no-bomb run and Remilia is a big jerk.