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)