I'm just gonna speak from my own experience here...
For me, when I was reading a lot of manga, I read more BL than your usual shojo romances. Actually I don't think I've read any shojo romances past the first chapter. This was mostly because I never identified with the main girl and never saw the appeal of her love interest. Then when my friend introduced me to her yaoi/BL collection, I remembered thinking, "ooh, naked guys. nice." So yeah, I'll admit it, that's what initially grabbed my attention. But on another level, sometimes I find myself identifying with males more (at least the fictitious ones). I dunno, that's just me on a personal level, anyway...
As I read through her collection, I found charm and humor in one series (Sex Pistols, called Love Pistols in the North American translation), and this pretty much set me on a path to read other BL titles. And as I found more series to read and as I got older, I found flaws (over-usage of the feminine/masculine pairing, other tropes, shallowness in story and character, etc.) and I started to wonder if all of it was just fluff or smut. It wasn't until I came across the works of Est Em, and to a lesser extent, Natsume Ono, that I learned BL/yaoi can be very deep. (I fecking love their work, btw, and they are huge influences in my art) So it's because of them that I am now very picky with my BL and romance in general. Their work also led to the creation of my own male x male romance series. It's not cutesy, it's slow-going, both guys are very masculine, but I love it nonetheless.
So TL:DR version: BL appealed to me because I (used to/sometimes/usually) identify more with fictitious males in a romance than the females, I've found really good BL/yaoi stories and they've set a bar to compare all romance stories I come across, and the basest one of all which I'm not ashamed to admit, I like looking at handsome men.