It's actually quite simple: Mechanics versus content. This doesn't just apply to FPSs, but Racing and sports games as well.
It's incredibly easy to make a Shooter "fun," or more accurately, it's easier to make games in other genres "not fun." All it really entails is putting together somewhat competent mechanics. Compare this with, say, an RPG, a platformer, or an adventure game, where the actually context of given actions are what makes a game enjoyable.
This is because other genres live or die by the sum of their parts, where as a shooter can at least somewhat atone for everything else with fun mechanics. This isn't to say that any genre is inherently better or capable of more depth, due mind you, but it does have an effect on what people are going to look for when they want a game that guarantees them (or appears to guarantee them) at least a small amount of enjoyment.
It's incredibly easy to make a Shooter "fun," or more accurately, it's easier to make games in other genres "not fun." All it really entails is putting together somewhat competent mechanics. Compare this with, say, an RPG, a platformer, or an adventure game, where the actually context of given actions are what makes a game enjoyable.
This is because other genres live or die by the sum of their parts, where as a shooter can at least somewhat atone for everything else with fun mechanics. This isn't to say that any genre is inherently better or capable of more depth, due mind you, but it does have an effect on what people are going to look for when they want a game that guarantees them (or appears to guarantee them) at least a small amount of enjoyment.