Mostly because Burning Crusade was the last expansion to contain gated content which provided an artificial sense of elitism because it showed you managed to get further than other players (though that was possibly only because of guild poaching, favors, luck, backstabbing, etc). Wrath also introduced separate raid sizes for the same instance, the dungeon finder, and made Retribution Paladin acceptable for about 72 hours (players weren't having any of that.) When games and social activities become popular everyone puts on their hipster glasses and talks about how they were into it before others were, and Wrath was the upper spike on the games popularity and player populace so they're the prime targets. They still talked smack about Wrath up until 4.2 nerfs and 4.3, then Cata joined that club.
Basically anonymity + internet + WoW = complete ass. Well, unless you're like most of us who just don't give a damn.