Although I have the feeling that noone is reading this, I still write it. It is about why Yukis other singers besides of Kalafina are not known outside the Yuki Kajiura Fan Community, regardless if they are working with Yuki or not (best example Kaori). My main Focus will be on Kaori itself. First of all, all of Yukis songstresses and singers are talented singers on their own, and the likeness of their voices depends on any individual person. But only Kalafina manages to be popular outside of the Yuki Fan Community. For those who deny this claim, I ask only for calling me a singer which name is mentioned by a song made from Yuki Kajiura and which is not Kalafina. I can name only Calling from Baccano, but let be honest, most person does not even know that she is the singer of this song. Tv Tropes, the site which have information of popular themes in entertainment medias, does not even mentioned Calling on her awesome Music part for Baccano. So, why is Kaori so unpopular, although she has made many songs without Yuki. I do not want to make Kalafina bad. I love their Music, although it does not seem so, but I will always love Kaori more cause I never have the feeling she is dependend on Yuki as Kalafina. Kalafina have the reputition, because of 3 points, first, their songs are written by Yuki Kajiura, which is an easy access for some anime fans, second, the anime for which their songs are made are really popular, namely Madoka, Black Butler, Fate Zero, Sword Art Online etc.., whereas Kaori has mostly made music for otome games, which are far less popular than Anime itself, yes she even does not sing the opening theme for the anime adaptation of Amnesia, of which she has song many songs in the video-games. Kaori is not worse than so many singers, which have more success than her, she is even better than most. Objectively, there are just a few singers better than her, but she is still one of the best and deserved better.
Kaori is not unpopular. She's unknown. I've said this before. Kaori needs to tour and tour continuously. Kalafina was a niche group with their first album. Getting out and touring for the last 8 years has built up their fanbase and general popularity. They tour when they don't even have content to promote. They are re-creating their content over and over with their tours. What has Kaori done ? a few small lives here and there. In a 10 year solo career she boasts only 10 live events / concerts (not counting invitation events, which are no more than 10 as well). And none were over 1000 in the audience. Scandal is the other group I follow that I hold up as a touring success. This year they did a 47 prefecture tour of small to middle sized venues with only a "best of" album to promote. They tour. They build their audiences. Kaori doesn't. I don't know whether that's her decision or her management's. Also, she doesn't have any of her concerts filmed. This cuts down any further chances of her promoting herself. It could also be that she's comfortable with the level of success she has.
I agree with @ghostreaper but also in that she's unknown, I think anime has something to do with it. Though Kaori does anime and game songs, she doesn't really have anything mainstream under her belt. Kalafina, on the other hand, has songs for Madoka, Fate, and Black Butler- all of which are mainstream anime, which immediately gives them an audience.
Then again, if you're going to target Kaori's unknown-ness due to her lack of concerts or live performances, what about ClariS? They've been around nearly 8 years and have done what, three lives in their entire career, all of which were in the last couple of years? And yet, their first major live was a one-man in Budokan which, if I recall, sold out. Touring helps, for sure, but I don't think that Kaori not doing that many lives is the real reason here.
I'm not sure what the reason actually is, though. She's talented and still releases material at a pretty decent rate, yet it just feels as though nobody notices that she's there. I don't get it.
ClariS is an exception to the norm. I don't know them or their history. Can't speak of them. But, from what Seasonreaper says: they are an exception. For every one else: it's the grind of touring and performing. A song on a popular anime or tv show or movie can only take the average performer so far. It has to be supported and promoted.
Although I have the feeling that noone is reading this, I still write it. It is about why Yukis other singers besides of Kalafina are not known outside the Yuki Kajiura Fan Community, regardless if they are working with Yuki or not (best example Kaori).
My short answer is, everyone is different - Kaori Oda also has her Sound Horizon work, the radio shows she has done and her time at Anipara Ongakukan. Chiaki Ishikawa is also a lyricist and although only started doing solo performances in 2011, she has done 3 this year at Mt Rainier Theater Shibuya (around 1000 capacity). Eri Itoh has done singing voices for Japanese dubs of Disney animations and also does musical theatre - I'm seeing her in the title role of the musical Mother Theresa.
These singers may not have huge numbers of followers on Twitter but have their fans and are respected by their fans and peers.
Still pretty interested in this, because her voice is great.
I do wonder if Place being her first album didn't help her career much. I'd say a first album is very important and, well, Place is honestly not all that memorable. There's nothing really wrong with it on a technical level, but it just doesn't have all that much staying power, especially compared to say, Colors or Gift.
Kaori said in her tweet, or was it the blog, that there were licensing "issues" with some of the songs from her live, so they were excluded from the CD.
Place was more of a compiliation of all her singles, with a few of her own songs added in. I believe Spacecraft gave her an album deal to prevent her from running off with her newly formed band Another Union, which promptly died when her album was released. Yeah. I believe it was a bribe
Metamorphose was more of a movie project thing than a long term group. It was a one off thing. Kaori is constantly adding and deleting things from her twitter tag. Another Union is no longer listed either. Wild Arms was listed for a while. It's gone.