Because most people here don't have an issue with the game, and if they do they don't come on here and complain about it causing an abundance of threads.
It is part of the nature of most game forums.
People for whom the game works and who enjoy the game, are spending their time playing and having fun with the game.
People who don't like the game or can't get the game to work, are spending their time in the forums reporting their problems and complaining about how they don't like the game.
As far as paying for any DLC for any game, no matter how much they charge, some people will complain it is too much. Even if they release a free DLC, there will be people who complain that they need to release an update that adds content to enhance the game and make it more "fresh" even if they have to pay for it.
Players have legitimate complaints, lag for example, it makes the time I spend playing this game frustrating and unenjoyable. I, along with others, have tried to get the problem addressed but it seems most threads turn into a network engineering debacle and I have yet to hear an acknowledgement from Treyarch that a problem even exists, which is even more frustrating.
The fact that the dlc is $15.00 is irrelevant to me, I can spare $15.00, I just choose not to purchase it. I can't justify to myself spending more money on a game I'm playing less and less of because of the multiplayer stability.
It seems odd to me that so many would buy the new DLC considering the forum is so full of threads on how messed up BO2 is and needs to be fixed.
The large majority of people who buy it, probably don't even frequent here...
This place is only a small pond.