I looked into this a while ago, and from what I found, incest is more of a big deal in cultures with a strong Judeo-Christian influence. In other places, like Japan, it wasn't considered as significant; in fact, there were many cases of marriages between very close relatives, even recently incestuous relationships between mothers and sons because the father was absent wasn't that uncommon...
Another simpler explanation is that in fiction, an incestuous relationship is more exciting/kinky/hot than a normal one. Just look at the porn market (both Japanese and Western) and you'll see it's full of incestuous themes.
Oh yeah, I was shocked to find that out too. It seemed some of those housewives thought was that having sex with their son would keep him from getting distracted by girlfriends and help him focus on his studies, and they were also satisfied because the husband was always working, so they both benefited... I thought it was just hentai fantasies, but it seems it wasn't so far from reality...( ̄∇ ̄")

Why is is incest so common in bl/gl? Can someone please explain cause it's pissing me the fuck off