Itch does not boost arbitrarily any games, except the ones in the featured and maybe the fresh list. At least that is what I understood from several threads in community.
You will notice that many comments on the popular games have links to youtube videos. So there certainly is a connection. An influx of popularity by secondary content creators. In other words, game play videos that are popular on youtube will result in games being popular on Itch. But the other direction is also true. Games popular on Itch will be streamed more often.
And since usually 90% of page one in popular on Itch have some horror theme, that is quite telling. It does not mean that horror is popular as such, but the popular horror games sure are more popular than most other genres.
Also, what you said. Who is gonna watch someone play the millionst iteration of a child friendly platformer game that suspicously looks like you already have played it. But for that matter, sometimes I think every second game here is a fnaf clone.