XxXFl0oD3RXxX said:
You're right. It didn't get the popularity it currently has by being the game it is. It's popularity spread through artwork.
ZUN himself said that Touhou is what the fanbase makes it. So artwork and doujin are part of Touhou, ergo, these artworks don't make it "look" popular; the artworks being popular is Touhou being popular.
I think the music and remixes also had a lot to do with it along with crazy videos like this one.
Suika was the first touhou character I ever learned the name of.
I learned of her from mugen videos back in late 2007 soon after getting a youtube account and wondered who is this giant drunken loli oni and downloaded her mugen character.

As for Zun on foreigners getting touhou it has more to do with legal issues vs anything else.
Supposedly it's a hobby of his and he wants it to stay such.
Rumor has it he works as a programmer at Konami.