Just putting this out there as a bit of speculation, steam did plan on allowing it at first then covid happened. That deadly diseases are a major part of the game may have made them hesitate. If they were already on the fence asking for removal of content that very well could have tipped the scales. Also a major enemy did end up being a crazed feminist... so yeah.
Basically three landmines all in one game, steam probably didn't want to risk being the platform to sell this.
Also lets remember steam never wanted to be a adult games provider, that was kind of forced on them do to the whole announcement of banning anime game that was released without ceo approval. Many seem to think the rules for what they allow are random, but aren't most of the ones they ban the bigger more well know ones, the ones that dont really need steam and can still sell well elsewhere. Steam and Nintendo both allowed games they previously wouldn't have do to the extreme censorship PlayStation and Xbox were doing, they were giving the market a place to exist for a time and get back on its feet, not a permanent platform. So they push away the games that don't need them. At least that's how its looked to me.