People may forget that long ago, but Call of Duty 4 didn't sell itself with it's multiplayer mode -- it was the awesome campaign/single player. The multiplayer just happened to be good and addictive and caught on.
Multiplayer games are impossible to predict -- the market dictates what becomes popular and long lasting. There's been piles of 'flash in the pan' multiplayer games that were marketed as the next big thing, and some where the multiplayer was added as an afterthought (Gear of War) and ended up really catching on. A multiplayer only title is a gigantic risk.
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