I could give an answer, like 'It was the youtube videos, the cute fumo meme, the characters and their relationships, the deluge of images on 4chan I never thought to investigate'. All those are true, even if they aren't the thing which made me decide to look into Touhou. What really drew me in?
You guys. The fans.
In an era where fans are continually milked by companies or are sanitized to appeal to investors, Touhou still feels like fandoms back in the 90s and 00s. There's a sense of playful fun and adventure. People aren't trying to prove their devotion to the source material by increasingly tenuous theory-crafting. There's no one scrubbing sites of any kind of criticism. And definitely no massive DLC pack or five year content map with a battle pass. Its a bunch of dudes and dudettes who enjoy something, who enjoy talking about the thing. Not some drama, the game's glorified marketing or wiki bullet-points. The characters, setting and story are at the heart of Touhou like very few games today, and the fans' love reflect that.