Why do you think that is?
Critics used to be people that went to school and knew a thing or two about making movies and the general technical details behind them, so they would have an informed view of a movie and they could easily see if the balance between story, actors, pacing and camera work where good enough, but of course their own opinion of the movie itself would always be a matter of debate because it is their own taste after all and we all differ in that one. So a critic would simply bring up points of "maybe this movie could had done better with different actors" or "the pacing was too rushed for a thriller" and those criticisms would be valid because they might be based on facts such as past thriller movies having a slower pacing in general and actors who had a history of being bad, but going into the "I don't like the story because it is boring" territory would make it more of an opinion rather than a criticism because it's not based on something factual but merely emotional.
We now jump to current day where you don't need anything to be a critic and you can see the problem. Not saying that every single critic on those sites is a random fag who just happened to have the critic label just because the idiot have been writing rants for a while now, but it does happen a lot, just click on their names and you'll see that all they have to show is that they have been writting for other trash sites but nothing else, no merits of anything or any mention of a cinematography school.
Now that we know that lots of current critics are trash because they are ignorant and emotional on a profession that requires education and a cool leveled head, we jump into the horror genre. Just as any other genre, there doesn't seem to be a consensus on what does horror actually should be, for some it's cheap jump scares, for others it's the psychological torture, so we already might have a problem on a genre we can not define that easily. If critics focused on the pacing and general intention of the movie itself, it would make sense that psychological horror must have a slow pace and focused on story and scenery, a gore fest horror would focus on effects and fast pacing action, etc, so unless whoever watching the movie understand all of this and can make an educated techincal reasoning on the movie itself rather than "I don't like it because blood makes me nervous, so horror movie is trash", we'll always have huge discrepancies between horror fans and critics.
Any other instances where this happened?
Basically everywhere lol, in movies it happens a lot in any other genre not just horror, like the retards complaining that the Godzilla movie had too many scenes of monsters fighting each other, or the critics bashing Sonic because how dare they not make a deeply philosophical movie for kids who like blue hedgehog that goes fast. In other media it is the same thing, like videogames and its critics, journalists and reviewers who are obviously idiots that either got paid for a good score or have an agenda against certain companies or subjects.
TL;DR : It happens because we lowered the bar and anyone can be a critic now and it happens everywhere because there is always something to review or complain about.
