It’s niche, I suppose. It’s not this hyper obscure franchise, plenty of people have heard of it, but the amount of people interested in it is far less.
I mean, it's an indie game. No matter how popular it is, it is gonna be niche because of that. But despite that, Touhou kind of is like
a juggernaut in terms of how many people actually willingly engage with it (in Japan)
There was this viral tweet a few years ago, about FGO, where a person was telling a story about how he went to visit his middle-schooler cousin and asked if they wanted to play Pokemon.
"Ehh, isn't that a game for little kids and old people? Nobody at school plays that these days."
"What do the kids play?"
"Fate/Grand Order."
"FGO..."
"All the boys realy like Mecha-Eli"
"Mecha-eli..."
You also got words from Japanese comic stores-- not big book stores, where they say that Touhou fanworks tend to sell out faster than anything they else they carry, and that the primary customers are young girls, aged 8~16. Touhou isn't this weird niche thing that people don't engage in, it's a cultural super-giant that has inspired countless people to pick up a tablet and draw, or learn how to program, or compose music.
It's the Indie-equivalent of dragon quest