As much as people hate on shooters, the genre has evolved more than others in recent years. Halo: Combat Evolved marked the successful transition of the genre from the PC front to consoles. Halo 2 brought a successful online shooter experience to consoles. What the first two Halo games were to alien warfare, Call of Duty 4 was to realistic man to man shooters. Following a generation where World War II shooters were all the craze, the effective transition CoD 4 provided to modern combat should also be highlighted. Quirks like kill streaks and perks served to freshen up the experience even more.
For all it's done as the most recent franchise to elevate the standing of FPS games, CoD has also contributed to the genre's recent stagnation where quick cash-grab sequels are encouraged. It certainly doesn't help that the franchise has been running on essentially the same engine since 2007. Other than that, I agree with RubenCrapton. Shooters provide quick, easy, and shallow entertainment for the masses. Although this negative stereotype can't be applied to all shooter franchises, more and more are taking the easy way out as defined by Activision and its yearly Call of Duty sequels.