Considering that the biggest bookseller of all, Amazon, has a separate category for LGTBQ, it makes only sense that eventually Tapas did as well.
Having more genres is simply to increase convenience. Otherwise there would be only two genre, Fiction and Non-Fiction. Imagine how aggravating that would be as for a buyer in a book store? "All our cozy mystery titles are on that half of the store and are sorted by title. Good luck."
Instead Barnes & Noble has Mystery, Thiller, Science Fiction, Manga, etc. even though they are all Fiction. That way the reader can get right to what interests them.
Having BL as its own genre just makes it where readers can get to where they want to get to faster. The less difficult it is to find what you want to read the more time you will spend reading and/or buying.
EDIT: We would love to see more genres in general, because as it stands right now, it's hard to find the content we want to read. Search is largely borked and Tapas only offers 11 genre. Amazon's books section offers 37. The harder it is to find content relevant to the reader, the less time they're going to spend on site, which is a lose-lose for everyone involved.