It's the same phenomenon that creates pop music, pulp literature, prime time TV, summer blockbusters, two-party voting systems, and AAA games. The mainstream tends to be safe, non-threatening, and appeal to a wide variety of audiences. There may be a genre/topic/idea that better suits people, but it takes work to find it and not everyone wants to put in that work.
Not everyone will like Thelonious Monk, and the same people who like him might not like Americana or electro house or black metal. But no one complains about smooth or cool jazz. No one complains about mainstream country. No one complains about techno, no one complains about hard rock on the radio. They're watered down, appeal to wide audiences, and are non-offensive in their broad market. It takes no effort to like mainstream culture, and it doesn't demand anything from your intellectually. You're going to like the Billboard Hot 100, or at the very least you're not going to hate it, because it's been manufactured to be liked by everyone.
Same thing with high rated comments in a lot of cases. While the poster might not actually be saying anything, might not have a point, might not have any intellectual depth to their comment, their comment offended no one, was easy to understand, and can be appreciated at face value without needing to think about it. If you can get people to like your ideas without making them think, you're going to be popular. If you make them think, they have the chance to decide if they like what you're saying or not, and if you make them think, you give them the chance to decide that they don't like you.