How did Touhou get popular?

While it may not necesarily be mainstream, Touhou seems failry popular in mostr fanart cirlces. How did it get so popular in the first place? I know it is probably the characters but other than that the games are nothing more than bullet hell shooters with fancy patterns. Still, how did it get so popular thaty I see fanart, fan games and doujins all over the place?

Ahother question, what if Touhou went mainstream?