itch.io

Or is adult just restricted for Sexual games?

https://en.wikipedia.org/wiki/Video_game_content_rating_system

The rating system on itch is not a rating system. But basically it is AO plus the M that might be considered "NSFW", and even T, if there are any that could have T, while also being considred "NSFW".

So, no, it is not restricted to sexual games, but because of the nsfw aspect, it appears so at first glance, as that one starts as low as some nude pixels. Also, there is the meaning in context of tagging and the tag "adult" is used as in the term "adult toy store". 

So what defines a 'popular' game?

Clicks. The algortihm on itch adds some magic to it, that they keep secret. But it also involves a boost for new games and very likely mechanisms to not count artificial clicks.

I blame the youtubers. They make playthroughs of horror games and send their viewers here. I do not say this is a bad thing, but it surely boosts popularity of the genre.

To me it looks like the default "Popular" view shows games that have the most activity.

"New and Popular" shows recent games with activity.

"Most Recent" shows recent games regardless of activity.

"Highest Rated" shows games with the highest star-ratings.

So, based on that it seems obvious why a certain type of horror game dominates the first category:

• They are short enough that youtubers can play the entire game in one video.
• They are sensational, but still allowed on youtube
• People tend to just play them once and then move on, which is why they don't have as big of a showing in the Highest Rated pages. 
• In contrast to the above point, people do end up forming a "trend train" where multiple youtubers line up to cover the same game, and their fans in turn go play that game. This gives the games a little more staying power in "Popular" than they have in "New and Popular"
  

To be fair, you can find plenty of horror games in all of the categories. I just think that the youtuber cycle is hitting just right for the titles they cover to be floating in Popular for a good amount of time (but, the titles don't usually have enough staying power to end up as Highest Rated)