Softbois are quantum-gendered, where they are non-threatening, and all characters have both feminine and masculine traits. This allows the (overwhelmingly female) reader to imagine themselves as either side of the pairing, as the Bitch or the Butch. Increasing in the western or globalist world, the "bitch" role allows a female reader to imagine "giving in" to the traditionally feminine roles and urges.
The "butch" role allows the reader imagine themselves as that character where they can be safely sexually aggressive against men without being seen as a slut by society - more importantly by mean girl Becky.
The audience can become either part of the fantasy since the "grass is greener"; either role is free of the societal expectations women are subject to. Additionally characters have none of the hangups of the readers regarding their own sexual characteristics (loldongs) so the softbois can behave as the female readers believe they'd be able to behave as men, and there is no direct comparison to be made between the audience and the characters.
tl;dr Yaoi is popular with women because it allows them to have a romantic fantasy without having to compare themselves to other women, and allows the fantasy to include a character engaging in 'feminine' behavior without societal expectation. Girl on Girl porn is the mirror reflection where it gives men a scenario they can project themselves into without male threat, and allows the fantasy to include a sexually assertive woman without threatening the viewer's masculinity.