The assumption is that we ought to choose one or the other and if we did we'd be better off. More subtly, bisexuality is simply erased by monosexist assumptions. Bi invisibility is linked both with homophobia and with the way that monosexism erases bisexuality. It also comes from the relatively recent beginnings of the bi movement. Some people have suggested that the bi movement is about ten or twenty years behind the gay movement in terms of numbers and resources.
Many people are still completely unaware that there is any such thing as a bi movement.
Also, the bi community is relatively under-resourced compared to the gay community and even the lesbian community. But lesbian and gay organisations haven't all welcomed bisexuals. 4 Biphobia from the lesbian and gay communities. Bisexuality has been a contentious issue in the past for lesbians and to a lesser degree gay men, and continues to be an issue from time to time. Sexism, from gay men about women and from lesbians about men, can influence their view of bi people, if they see the bi person as somehow contaminated by the other gender.