About 2% of all games here are nsfw. About 7% of all games here are horror games. About 14% of all games here are pixel art.
Sooo, what's going on?
The average nsfw games have a lot higher quality than the hobby and amateur games uploaded. Like, really a lot higher. AAA studios are not really releasing adult games. So it is mostly an indie thing. And Itch is the place for indie games, be they nsfw or not. But combined with the higher quality and the fact that you can get regular "Indie" games with even higher production value on Steam, the apparant popularity of nsfw is quite high.
A similar thing can be seen with horror. Streamers like those and page 1 of popular is usually 90% horror. But horror itself is not actually all that popular on Steam. But in the indie sector it is. And again, Itch is the place for indie games.
As for being a preferred place, you can't upload a work in progress to most stores. They want a finished game. That goes for all kinds of games, of course. But the more finished and successful nsfw games will often be published on Steam.