It's a chicken-and egg problem -- the publishers won't take it because it doesn't sell, the bookshops either won't stock it at all or put it in the non-fiction under "gay erotica" or "gay studies", where it doesn't sell to people who are looking for romance, and the people who would buy it if they knew about it are looking in the romance section and not finding it...

I think one of the reasons for the emphasis on science fiction and fantasy in the ebooks is that sf&f readers have been able to find gay romance sub-plots in mainstream sf&f books for a long time, plus slash fanfiction started in science fiction fandom. So a lot of the people now writing professional gay romance for the ebook market (including me) came out of a science fiction and fantasy background.