Iām guessing itās just much easier to represent the idea of LGBT with monosexual gay characters, because I think it all boils down to seeing same sex relationships in media, not specific orientations.
I also think thereās the stereotype and stigma of a bisexual being unable to be content with just one gender or partner, like a character canāt be bisexual unless they date both genders in whatever franchise they are in, and thatās definitely a shame.
But, you say that bisexuality is more common. I can see that that might be true, but itās also factually researched that a clear majority of bisexual people date the opposite sex. So on that regard, would I be happy with majority of LGBT representation being bisexual people in opposite sex relationships? Hell to the effing no.
When you say that bisexuals need representation, which they do, Iām extremely vary of companies, writers, artists, cashing in on āā progressiveness āā by adding bisexuals to their products
a) who donāt ever have same sex relationships and never imply having had them
b) who do have or had same sex relationships but they arenāt anywhere on par with their opposite sex relationships in terms of importance and functionality
c) who donāt even mention their attraction to the same sex anywhere in the product, but their writers announce their bisexuality regardless of that
Iām not trying to erase someoneās sexuality, but Iām still vary of this kind of phenomenon happening. And I think a lot of writers acknowledge these things they need to look out for when writing bisexuals, and opt out from writing bisexuals, especially if they are characters whom they have a specific love interest for, instead of several.
All in all I think itās a mix of outdated stereotypes and stigmas, as well as the ease of representing same sex relationships with monosexual characters.
But I have to say, that you imply that gays are overrepresented, I donāt really see it. When saying gays are overrepresented, you say we donāt need more, and that we should be content. I donāt like that. I think itās more fair to say we need more bisexual representation, not less gay representation.