The majority of the people who use the boards are hive minds.
I see one post talking about how broken a champion is, and it has 10 downvotes. A few days later, another post talking about how broken the same champion is has 20 upvotes. People here are incapable of forming their own opinions. Also, they're just rude and close minded in general. Mods seem to be people that exhibit these characteristics more than the average boards user; that's really all that sets them apart.
TL;DR the boards are more toxic than the actual game is.