I dunno, as far as I know, it's always been popular since the middle to late 2000's. I suppose it's boomed with the advent of comic websites, either scanslation sites or original comic hosting platforms.
I'm sure many girls (and guys) like it because you get double the D. But a lot of them, especially Japanese ones, have main characters that can be self-inserts for the reader as they're usually more feminine and androgynous. And I think a good number of them are presenting a fantasy, even if it's mundane, like all the high school ones (omg, so many). It's a different flavor of the Harlequin romance novels, full of tropes meant to titillate and satisfy; your typical series not meant to provoke thoughtful discourse or reflection of the plot and characters.
I'll admit, I will often check out a series with the BL label just to see how trash it it.