The problem is most of the game developers think from the traditional male perspective, meaning there is a guy, and a woman(or multiple women), then he wants to get into her pant, then he does a series of activities to make it possible. It's honestly quite lame how women throw themselves at MC in a lot of games. That's the core of most of the games here. A lot of lesbian women don't like that approach, they want it to be more emotional, the story has to be evolved properly, character development is important as well, then sex be a part of it to share the mutual feelings. I am not saying lesbian women don't enjoy sex, but they do it along with a good story. Of course it's not possible to generalize everyone, but that's what majority of them enjoy. This can't be done with a small game, it needs a lot of texts, but not too much to make the reader bored. so the developer has to manage it cleverly. So I think it can be challenging, but it's worth because it's still an untapped gold mine. I don't think the audience is low in number, but they are not fans of poor quality games.
True, it's also possible to extend further in technical point like the engine used.
I noticed that a big majority of games made under renpy, so vn with varying range of gameplay, tend to focus on males perspectives. Can't really tell if it's because the audience and players prefer to play as male characters in these type of games, or if it's because majority of said players are male and find it easier to associate to a male protagonist.
Same thing for devs I guess, easier to tell a story when the character you create is the same sex as you.
While on the reverse, rpgm games are most often than not with female protagonist, but are also more focused on smut, with mostly males characters, letting the emotion / build up / relationship part a bit out.
Maybe because rpgm give a more external feeling to players, like they are not playing as this female protagonist, more like external watcher controlling her action. A bit hard to explain...
As for the lesbian part, I can't really say, it's not uncommon to see lesbian scenes or plot in these adult games, but it's rare for it to be the "main" romance plot.
Maybe (saying that a lot ^^') because it's games. A quite different media compared to films, books or doujins.
Everybody is different but for me, I love lesbian content in movies, video and doujin (even to an "only-lesbian" level for videos)
For VN I prefer male protag and for rpgm I prefer female protag, with a bigger focus on hetero content.
I won't continue talking about other themes like futanari, trap, sissy etc ^^'.