How can i balk at reading things with objectionable content because I like the F Plus? Because I find certain subjects (pedophilia, bestiality, incest) too vile to willingly consume media which endorses them. It's not like I'm going to go and burn all the Nabokov books I own because he wrote Lolita, because Humbert Humbert is a sick, disgusting man. I enjoyed Top Of The Lake because even though it's a mystery about a small town and the rape of a child, it doesn't portray that rape as morally neutral or even good. But a series in which the main relationship is an incestuous one and it's all a-okay disgusts me and you're damn right I'm going to judge. That it "spawned an anime adaptation, manga, videogames, radio shows, a lot of merchandise and was even emblazoned on several train cars" is bizarre and might be worth mocking or investigating if it wasn't already about incest. (e: underaged incest no less!) I think I agree with you that the popularity of it is the weirdest part, but it definitely has competition!
Regarding trilogies, both of those explanations seem reasonable to me. Thanks!