Honestly? I've not heard a single person outside of Smashboards ever bring up Reimu. I guess I kinda sorta understand her appeal, but she's leagues more popular here than I would have expected.
You'd be surprised how much support she's gotten lately. Exhibit A: The Reimu For Smash account, and all 5,158 followers.
Reimu For Smash (@ReimuForSmash) / Twitter
Plus a decent amount of videos regarding her and Smash, from moveset concepts to music wishlists.
There's actually a lot of Reimu support going on outside of here if you know where to look. Heck, Touhou and Smash is actually what led me here in the first place.
As for
why she's sprung up so much, I think it's a combination of A: The doujin leak acting as a bit of a Trojan Horse to get people interested, and B: ZUN's statement on wanting Touhou in Smash making people realize "Oh
%$#@, this can actually happen!" Yeah, everyone under the Sun wants their guys and gurls in Smash, but ZUN's known for doing his own thing and avoiding excessively commercializing Touhou, and people used to think he'd be the main issue regarding Touhou in Smash. Touhou's already had a huge fanbase, but it tended to occupy different circles than the Smash fanbase. These two events brought them closer together, and the Touhou fanbase just spilled into Smash's.