And it's a good question. I think most of the reason is that most of the people (not all) become obsessed with it and start to copy it. Soon everyone would do fortnite dances and they were usually pretty obnoxius.
Another reason is how much people would argue over it. Not only that, they would belittle others because they didn't have a victory royal or didn't know a dance. Once again, this is not all formats players, but most of them.
And finally, how Much it took over every single social media platform. Even If somebody hadn't really cared about fortnite to begin with, their youtube recommended was flooded with Fortnite. A lot of people didn't like this, and they got angry at it.
I personally just think it's overrated.