Re: Why does COD always get more sales over other better FPS games?
It's a fast paced game that provides a polarizing experience. As long as you see a bit of both sides, it'll pull you in. Kill streaks are are fun to use because of how destructive they can be. People die fast, giving opponents less opportunity to outplay you like one would in Halo 3. Large amounts of customization and it utilizes a mmo-esque leveling system to reward players.
COD4 and Youtube really boosted the popularity of the series. Game commentaries became popular around the 2nd year of COD4's release. Those were basically readily available advertisement. Back then, you searched on Youtube COD4 tips and you could stumble upon these commentaries. I've heard that COD4 is widely considered the best one because it was the series leap to the modern saga among other smaller details.