Because we still refer to them as LGBT characters.
They’re still a novelty. We get all excited (or angered, depending on your political/religious stance) when a character in a TV show turns out to be gay or bisexual. Let’s say, being attracted to the same gender is a recessive trait (yes, I do believe it is genetic), then statistically, gay or bisexual people should make up about a quarter of the population, yet in the media, they make up only between 1-5% of the characters in shows and movies. On top of that, in certain aspects of the media, they are wholly absent. Do you see the disparity between population and representation?
It has really only been since the 90s that it has been “acceptable” for openly LGBT characters to appear. In books, it probably started slightly earlier, but the controversy was still there (I could go on a whole tangent about late Victorian gay erotica and literature from that time to the present that used only innuendo, but I’ll save that for a different post). The point is, despite numerous states ratifying gay marriage and the movement really picking up steam, they are still “othered” by society.
Even within the media where portraying LGBT characters is growing, transgendered people are still nearly completely absent as are bisexuals. Bisexuality tend to be used as an excuse for a promiscuous character sleeping around and causing havoc in a cast of characters, but apart from being a trope, they are rarely seen. Marvel’s universe seems to be breaking through this barrier from time to time, such as with Agent Carter. If I remember correctly, Peggy Carter is bisexual, as is Steve Rogers (Captain America). I know in comic books, this is a lot more common and really took off because comics have often been seen as what I call “fringe lit” since the literati paid it little heed and allowed it to grow unchecked. This allowed for greater expression of different characters with varying sexualities and genders. Transgendered characters, much like bisexuals, are still used as tropes. Most are seen as RuPaul-ish, where they are still seen as men in drag and not characters transitioning from one gender to another or living as a man/woman. Why are transgendered or transsexual characters so absent still from media representation? Because they are furthest from the perceived norm. It’s as if the writers never spoke to a transgendered person before and decided to portray them based off only preconceived notions.
Asexuals. Poor asexuals. In this sex-crazed, sex sells world, they are absent for the most part and tend to be invisible. If a character doesn’t want to have sex, of course they have to have been abused or raped or impotent, and if they are asexual and in a relationship, then they’ll have to give in and have sex to please their partner, right? Wrong. Once again, this aspect of their identity is ignored all together.
What I aim to do in my own works is explore varying sexualities and genders in a realistic manner and give them the presentation they deserve. Some may wonder why they should bother to read or write about characters who are LGBT, but I often think about teenagers and how confused some of them must be. Is something wrong with me because I don’t feel the same way as the majority? Why am I female but feel as if I am male? I don’t want to be in drag, I feel male. Would life be easier for this child or even adult if they read a book or watched a show and connected with a character. Finally, someone feels the same way I do. Feeling alone in the world is one of the worst feelings a person can experience. Even if you are surrounded by supportive people, connecting with another person, even a fictional one, takes away that loneliness.