That I can answer for you. I myself have stories where there are characters who are homosexual, bisexual, transgendered, pansexual, polysexual, asexual, and aromantic. I would say 90% of my stories are heterosexual, with about 90% cast of heterosexuals, however, I mention characters or just a few random bits of others who are of the sexual orientation. Casual conversation, and there is no name for the sexualities.
I have my share of them in my worlds, because I believe if you are going to build entire worlds, you cannot tell me that there isn't going to be a single homosexual on that world. How unrealistic. I want my worlds to feel real and alive.
Now, I said I have your answer as to why they are not seen often or popular at that. I said something similar in another post on high fantasy, but it really applies to this more than my vague mention of it before:
In a world that is considered mostly heterosexual, what are readers going to relate more too? What will drive the sales of novels. Now mind you, what drives the sales of novels means how many people are aware these stories. How much you impact people with them. Doesn't always mean, what brings you the most money. I want to spread my novels far and wide, even if I really didn't get paid. So, going on.
Will readers really relate to characters that are homosexual, probably not, however, that depends on how well you have written them. If you want to keep 100% of the fantasy audience, heterosexuals and homosexuals alike, I believe that you should do a couple of things.
1. Never ever make their sexual preference who that character is. Did being a heterosexual define who Richard was int he Sword of Truth series? No, his amazing feats did. Just how Dumbuldore was defined by his amazing power as a wizard. And, Jane, your own written lesbian queen, will be defined by her rule of her people. Don't make a huge fuss over it and people will accept them. Because the moment you do, the story becomes who they are attracted to, and that: A. Sounds boring or B. Sounds like a romance novel or C. A homosexual only novel.
2. Do not name their sexuality. In the stories that cover my non-hetero characters, homosexuality has no name. There is no. "So, the village black smith came out of the closet the other day." or "The Lord's daughter is a lesbian." There should only be. "So, the black smith and his betroth, Riven, both fought off the King's men for trying to force our village to give up our resources!" or "The Lord's daughter is marrying the Princess of Tresfeldur! That lucky girl, but she will make a terrible queen. Always spending her father's money, she was!"
3. I oppose sex scenes, unless they drive the story. Recently read a story of a guy who was getting a bj from this woman. It wasn't necessary for the story. In fact, that scene didn't play ANY kind of role in the story at all. I was very disappointed. Sex scenes that you found in the Wheel of Time did in fact play a role in the stories. They were showing the characters roles in the story and how they would fall in love with so and so, how their relationships would workout or if they didn't. So, is a sex scene needed with your homosexual characters? That depends on your story, but there better be a good reason.
"The Prince had an affair with Lord Venner. What will the Prince and the people of Mul'loki have to say. You don't think we will go to war, will we?"
"Alice, the inn keeper finally bedded someone, I think she is moving on, now after year since Mellany died. The poor girl, she was so sweet."
Never make a big deal out of your character's sexuality and it will flow with the story.