Because for ages, but especially in the 90s, bisexuals were the devil incarnate in the gay community.
It was common for those just "experimenting" with homosexuality, especially women, to go out of their way to identify as lesbian then claim bisexuality when they found a Chad they wanted to marry and had to explain why they were riding Chad's cock.
For a lot of bisexual people, not just women but men too, it was a matter of survival to identity as gay instead of bisexual because one's gay friends would turn on you the second they found out you still had feelings for the opposite sex due to a fear that you A. were just "experimenting" for kicks and B. Would use being bisexual to basically get straight privilege and be able to pass as straight/have your homosexual experience excused as "going through a phase" and not held against you in a way that could ruin your life, once you resume a life of heterosexuality.
But for lesbians, especially those of the militant variety, being bisexual meant you were a traitor in waiting. Ironically, this was especially common amongst the lesbian/feminists who would be at the foreground of the TERF movement, as far as their politicization of their gayness meaning that you had to be 100% gay or else be a traitor.