With over 800 picture relating to hypnosis/corruption, Touhou is a popular source for this type of material. Yet, I haven't found a great source that explains what it is and why there is so much content from this series. Can someone answer what it is and why so much content?

What it is - A bullet hell game series.
Why so much content - and impossible to count number of characters. And sexy art for a lot of those characters. Also epic music. XD

Yep! Touhou is a bullet hell game! Intense stuff, I think I've only beaten perfect cherry blossom... And well, I honestly can't say why Touhou has been so popular, nor can I explain why it has one of the largest collection of lewd images/doujin/etc, but I'm thankful for it's size anyways!

Because Touhou has characters for every trope, archetype, species, height and cliche known to man.

There is a small overarching plot/idea, but since it is so vague writers are free to implement their own ideas, as well as flesh out personalities for characters.

The original art was also not really drawn that well, which also lets artists have their own take and put their own details onto the design.

The music, which if I'm not mistaken is what ZUN wanted to promote originally, was also somehow really popular for fans to make remix versions of.

Combine all that with cute girls


and you get a surge in popularity.

It's somewhat like kantai collection/love-life/school idol shows, where you just make a bunch of characters and at least some will appeal to someone, but since they're all part of the show it allows many different people to get into it.