Because "gamers" are a group of people that consider themselves to be in an elevated position in which they think they can condescend and consider other people's preferences and views to be inferior. In the context of video games, they find games to be "uninventive" and "unoriginal" and constantly bashes it, which also extends to the people that play that game and enjoy it.
There are many types of gamers such as this, such as the "nostalgia gamers", who are most known to look down on the newer games and constantly refers to the golden days while at the same time, looks at the mainstreaming of video games to be worse when gaming was more obscured. One very frequent type that are also making their appearance is the "popularity basher", who constantly bashes any game that is very popular within the industry and has a high reception from the general audience. They will, at any attempt, point out every single flaw they can find with the game, makes reference to less popular games and points out why these less popular games are superior to this. Often, they see the people that enjoy and praise that popular game, to be delusional, inferior in deciding what makes a good game or a "mindless zombie giving money to the developers for nothing".
OT: People hate WoW for many reasons (A lot can be found in my random ramblings above), but one that seems to stand out above all is the fact that WoW is seen as a game that "takes over your life". This is a common misconception and I find it appalling that people who bash people for calling video games out to cause violent behaviour, are also the same people that calls WoW out for "taking over your life".