Ok so I know who Reimu is but I don’t exactly know what Touhou is, can someone fill me in on why it’s so popular? I’m curious.
Attempting to be a succinct as possible: Touhou is a series of doujin (indie) games made by Jun'ya "ZUN" Ota that mainly consist of bullet hell shooters, though under the banner of official "cannon" media there are fighting games plus a magazine and half-dozen manga. The setting is the land of Gensokyo, a section of Japan sealed from the outside world sometime in the 1800's where youkai, fairies, and gods can thrive unimpeded by modernity. Typically, the games focus on the player character (Reimu, Marisa, or someone else) setting out to resolve some incident that threatens the stability of Gensokyo and defeating the people they meet along the way with attacks meant more to dazzle than destroy. The games are known for their nail-biting gameplay, colorful cast of hundreds, and exhilarating music. The games started in 1996 on the ill fated PC98, but after making the jump to windows with the 6th game the series began to increase drastically in popularity.
Touhou experienced a major popularity boom from the early-mid 00's thru the early 10's, though it maintains decent popularity to this day; note that statements about popularity are increased within Japan and decreased elsewhere in the world, but there is substantial cult followings worldwide, particularly in China and SE Asia. What really made Touhou popular was ZUN's lax policy towards derivative works, as long as you didn't crowd source funds and gave ZUN credit for the original work you could make whatever you wanted. This lead to a massive influx of remixes (sold on CDs), fan games, and fan manga, ensuring Touhou dominated ComiKet (Japan's Comicon) for a decade; it got so bad Touhou was forced to make its own convention, Reitaisai, which maintains comparable attendance, and even still to this day Touhou is consistently in the top 3-5 series represented at ComiKet. The Touhou boom ensured that its memes dominated early Niconico and YouTube, and Reimu and Marisa were even mascots of 2chan for a time. Perhaps surprisingly, this means a large amount of Touhou fans haven't played the mainline games; however, this can partially be attributed to the limited (legal) access to the games at the time, particularly outside of Japan. Now the most recent games are on Steam, so that's less of an issue.
The decline of Touhou coincided with the rise of new series competing for the same audience; mainly KanColle and Granblue which (if I may be slightly inflamatory) attempted to astroturf their way into Touhou's success. In short, Touhou fell of from a changing landscape of games and a growing market of consumers. Still, new Touhou games, both official and fanmade, manga, art, and remixes are made to this day, in quantities that put AAA games to shame.
Touhou's, and thus Reimu's, divisiveness and perceived obscurity in the Smash community can be explained by several factors. Smash tends to be a melting pot of game communities, so it's understandable that people may not be familiar with each other's interests. As well, until the Switch Touhou has seen limited exposure on Nintendo platforms, most notably a cameo by Reimu, Marisa, and Sakuya on the 3ds Taiko Drum Master, so many Nintendo fans could only be familiar thru outside means, though it can be inferred many Touhou fans like Nintendo as well (everyone that posts here, the various fangames lovingly "borrowing" from NES and SNES titles, etc.). Many Touhou fans also tend to stay within the Touhou bubble, some going so far to decry more mainstream Touhou exposure as "ruining the fandom," whatever that means to them. But most simply, because up until Cloud, Joker, even the Sans mii costume it seemed flat out IMPOSSIBLE that Sakurai or Nintendo would ever select Touhou, the ultimate underdog juggernaut, the most popular obscurity, the epitome of the doujin spirit, the internet's best kept secret, something only tangentially connected to themselves, to be represented in smash. I was an early adopter, voting for Touhou in the smash ballot, and there was disparate support here and there, some blog posts, a dream smasher write up on Source Gaming but not much; the difference now is there is tangible, credible hope, and that's why people are speaking out.
Sorry for the text wall, I know I said I'd keep this short but I think that's physically impossible for me. If you wanted to learn
even more for whatever reason, I'd suggest the Touhou wiki.
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I beat the **** out of my friends with this girl in Smash Infinite, so I want to beat the **** out of them with her in Smash Ultimate.
You sir have excellent taste in protagonists, Master Spark for life!
... My favorite usually by far are the designs that stray the most from the magical girl influence... 
