A think there are a few reasons why the COD series continually dominates in terms of sales.
COD4 was, I think by many shooter fans standards, a very well made and fun game that added a lot of what other modern shooters were missing. It was hugely successful and afterwards became a reliable brand that the average consumer felt they could get their money's worth with. And after that the ball just kept rolling, as it did with WOW, Guitar Hero, Rock Band, where everybody and, this is important, their friends had it. COD is still carrying that momentum and will only lose it once someone else makes a shooter consumers find more fun.
As far as the actual quality of the games goes, the COD multiplayer does a very good of stimulating the reward centers of the brain. You kill someone *reward* you level up *reward* you die, no biggie quickly respawn, kill someone else *reward*. I mean, that's the whole point of the killstreak reward system. The gameplay is fast paced, maps are small and usually little time needed to wait, which keeps you from becoming bored. A lot of hardcore gamers criticize COD for its lack of change since COD4, but if they changed these aspects they would completely lose the part of COD that so many people find fun.
So in short, brand familiarity and an addictive gameplay formula.
COD4 was, I think by many shooter fans standards, a very well made and fun game that added a lot of what other modern shooters were missing. It was hugely successful and afterwards became a reliable brand that the average consumer felt they could get their money's worth with. And after that the ball just kept rolling, as it did with WOW, Guitar Hero, Rock Band, where everybody and, this is important, their friends had it. COD is still carrying that momentum and will only lose it once someone else makes a shooter consumers find more fun.
As far as the actual quality of the games goes, the COD multiplayer does a very good of stimulating the reward centers of the brain. You kill someone *reward* you level up *reward* you die, no biggie quickly respawn, kill someone else *reward*. I mean, that's the whole point of the killstreak reward system. The gameplay is fast paced, maps are small and usually little time needed to wait, which keeps you from becoming bored. A lot of hardcore gamers criticize COD for its lack of change since COD4, but if they changed these aspects they would completely lose the part of COD that so many people find fun.
So in short, brand familiarity and an addictive gameplay formula.