I’m pretty sure most of the complaining about VNs on Steam has everything to do H-content. There was that whole situation/“scandal” thing with If My Heart Had Wings; from what I’ve heard, the whole selling point of Nekopara is the H, so removing it makes it seem kinda stupid; and I’m sure tons of people won’t want to buy an “all-ages” version of Grisaia. People who want H are going to complain when they can’t have H.
But Steam has a very good reason to refuse all sexual content: they want everyone to be able to buy their games. And it works 100% perfectly with stuff like planetarian because, like @LinkThinks said, it just makes everything so darn easy and convenient to buy and store the games. And as @Aspirety, it makes it so darn easy and convenient for companies to sell to as large an audience as possible.