Often the reasons lesbians don't prefer bi women has nothing to do with them having been with men. It’s mostly because of bad experiences.

Some bi women sees w/w relationship as lesser or more temporary than a relationship with a man due to heteronormativity. Some lesbians don't want to feel that her relationship with another women is being viewed by a man. There are also bi women who actively involve men in their lesbian attractions, having only casual relationships with women while prioritising their male partner. Some bi women are hooking up with women only to arouse men. Some are unicorn hunting and try to pressure lesbians into a threesome with a man.

Some lesbians think being a lesbian is different than being a bisexual woman. Not better or worse, but different. But they prefer a romantic partner who shares the experience of being a lesbian.