Because it's more about stats, dice-rolls and strategy.
Playing FPS is simple, move cursor towards what you want dead and click a button.
Anyone can play an FPS game.
Turn-based games require a bit of finesse and for the player to actually plan out his/her attacks on his/her foes.
Also, it's a much slower pace, some of us just like that.
I like fast-paced games as well, I love L4D.
But sometimes I want a game where it's not about twitch-reflexes.
