BBT would just draw eye-rolls instead of hate, except for one thing. There's a whole lot of people that fit the nerd stereotype on reddit. (shocking news i know)

So what's the problem? The show became the cultural touchstone for the mainstream's idea of what nerds and nerd culture are. And the characters are over the top stereotypes of what nerds are like. So now whenever a nerd is out there doing nerd things, he gets hit with the "oh you're just like that Sheldon on Big Bang Theory." This is not a flattering comparison. And so every time that phrase is uttered, the hate grows stronger.

Sorry for the overly serious explanation about a dumb sitcom.