Excellent question; I have dropped both "Alcatraz" and "Person of Interest" after a couple of episodes because of this problem.

Three hypothesis: a) The procedural formula is easier to pull off successfully with limited writing talent. b) Having a handful of permanent characters and the rest be one-offs is cheaper in terms of casting than a complex plot with many, layered recurring characters. c) We are atypical, and most people prefer the comforting, predictable procedural formula. (In particular, it allows for tuning in and out of a show missing episodes here and there with no great loss.)