In general terms I'd say, that it's just like with porn and that there aren't that many games, which appeal to the average women. SS is certainly among those which do, because of the writing, style and the mood, but I don't think there are that many girls who'd enjoy playing games where some rather creepy MC fucks his daughter/sister/mother or where you play a female character, that essentially gets turned into a slut or is involved into some NTR shit. The whole weg scene is focussed on men and there's certainly a lack of decent female MCs in general.