"Why are Romance and BL only popular online but not in the mainstream?"
it's popular in a niche sense, the online forums and webcomic sense is a niche scene compared to the mainstream scene let me give you a simple example:
Long exposure: has 72.2k subscribers, 8.4M total views and 84 episodes (this gives an average view count of 100k)
this one was the first bl comic I saw right in the front page of tapas in the popular section of this day tapas is the second biggest webcomic aggregate out there so now let's compare this to a mainstream aggregate platforms series.
Iron Fist: in Netflix, this was a failed mainstream series with an average view count of 5.8M gross view rate with only 24 episodes that means that on minimum the total view count should be 129M views
now I know it might look like I'm being unfair, oh pitting a Free webcomic against a production where thousands of dollars where spent, and that's just it tapas biggest BL webcomic brings in numbers that mainstream audiences deemed a failure. But ok let's try pitting them against something more up to webcomic standards
green arrow: 18,951 units sold in December of 2018 this was the number 100 most sold comic of that month while costing 4$ this means this comic made about 75804$ that month for the company
now long exposure does have a patreon but the profits are obscured so let's be generous and that the 1,010 paying him monthly are giving him his highest rate which is 12$ that gives us a total of 12120$ monthly
are you seeing what I'm saying, even this wildly popular comic for tapas is a failure in the eyes of mainstream numbers?
BL is a popular genre for a niche market, if you could get view numbers like Iron fist or at least green arrow number of money then it might have a chance but even the worst mainstream series bring in more views, more money and more people/ads than the most popular BL comic in this platform.
and this isn't even comparing it to mainstream online numbers, try looking at mainstream youtube if you open the trending page most if not all videos have 1M views. and some of them only have an hour on it, the most recent page isn't even at the level of a viral youtube meme at this point.
now just so I'm not misunderstood I by no means am saying that BL is bad or that they aren't popular, what I'm saying is that outside of tapas and some circles of the internet BL isn't even in the minds of people, they don't care for it or even acknowledge it.
now romance that's actually fairly mainstream since romances are in basically every aspect of media (even in men media) but that's because it's a universal experience that most if not all humans are hardwired to like, look at most blockbusters and there would be a romance subplot or a love interest of some kind, hell the last blockbuster without romance was I think Pacific Rim and people still wanted to believe they got together in the end.
romance is mainstream as you can get for both males and females.