Because it’s not adult entertainment the general targets are the “Shoujo” loving girls, who are homosexual.
In other words, it’s like “Houjo Manga”, but instead of beautiful boys, there are only beautiful girls in them, with the same storytelling, where there are lots of love triangles and betrayal.
It is also popular, where a lesbian girl takes heterosexual girl from her boyfriend and converts her to a lesbian, and in the end they get together.
The term and anime/manga style “Shoujo” started to get girls into Japanese culture in the 1990s.
The entertainment industries seeing the potential in the scene, started to produce products targeting the age group which “Shoujo” was meant for in Japan.
Some people say it original debut and rise in popularity can be deemed to the anime called Sailor Moon, which has first aired in 1992.
“Shoujo ai” started as a subgenre of “Shoujo Mangas” and anime, which was later taken up by adult entertainment industries, where they started calling it a sub-genre kown as “yuri”.
Anime and manga (Japanese culture) has huge popularity with both man and women, so when they targeted the young girl audience, they didn’t know the anime/manga style “Shoujo” will become so popular around the world.
Because of its popularity in the mainland and thanks to the internet and the large number of people around the world who love Japanese culture, it started to grow outside of Japan and is constantly getting translated to other languages.