Probably the story is what I'd guess. The whole idea of a dead nameless MC that jumps around possessing various characters as spirit probably isn't something most people can get into, or a Main Character they could "self insert" or fantasize as being. Possessing female character(s) probably doesn't help the situation either.
It's like you see people saying they want something different than the tried and true kinds of stories you see a lot of times with adult games, but in reality they really don't, or they just don't want something too far off from the tried and true. I mean, there's really no reason quality wise that this game shouldn't have more patrons, so the only other place to point the finger is the story.
Personally, I've tried playing this game numerous times when I see it updated here, and it's either I just end up deleting it without ever playing it, or I try to play and never make it far before I delete it. I think maybe one time I used the mod to see the amount of content that was there with the Devil and there wasn't a lot so I just deleted it. I don't remember when that was though, so I'm up for checking the mod again, and seeing if the amount of content with the Devil and the redhead on the banner image is worth playing.