A lot of people grew up being bullied, turned on a show that was supposed to appeal to them, only to be bullied again. 90% of the jokes are "hey look how fucking stupid these nerds are for liking nerd things." The remaining 10% is sexist or ignoring Penny's crippling alcoholism.

It was an attack on a culture of people, rather than the celebration of that culture it pretended to be. That's why people hate it.