I've wondered this myself and the answer that makes the most sense to me is the subject matter and the source material is too grisly, too grimdark, too fucked-up. I don't think it would get greenlit, its too edgy and dark.
The book is one thing, but the manga adaption takes the material and amps up the gore and perversion to 11. The mangaka made Mitsuko have a horrifying past where she was raped as a child by her stepfather all the time. The mangaka also made the gay guy a total femme drag queen stereotype, complete with a scene where a troupe of hardgay club dudes trick some guy into fucking them. It's weird.
And the mangaka reveled in showing close ups of cute girl's faces having bullets rip through their eyes and mouths, having their faces and heads explode inward in slow motion.
There are a lot of cool fights from the manga that I would love to see animated tho. Having the martial arts battle between Kiriyama and Sugimura animated would be great. It's actually a really good and well done fight.