Too be fair the bulk of Leftwich's playing time came before the NFL started manipulating the rules and making the game more passer friendly. Bridgewater is a lesser talent then Leftwich yet he sticks around while Leftwich lost his starting job after 3 seasons to Garrard and became a career backup.
The explosion of passing stats was underway before Leftwich came along. In 2004, when Leftwich was throwing 15 TD passes, Peyton Manning threw 49! A career peak of just 15 TD throws is very poor, especially for an immobile pocket quarterback. In '04 nine QBs threw at least 27 touchdown passes and 22 threw more than Leftwich's "career year" total of 15.