Because she wasn't the title character.
It would have been actually really neat if the author subverted the expectation wherein the title character wins at the end, but he was a hack so that didn't happen.
Because you can't show little girls smoking on tv.
Because the mc is a sister lover, he even states to avoid any confusions like this. He only dated kuroneko because who wouldn't say yes to some random cute girl? He's a faggot for breaking up with her though.
Maybe not best, but solid taste nonetheless.
She's a miracle of the universe.