Tv shows into movies?

why are not more tv show franchises turned into movie franchises when they become successful that's how star treck progressed. why is it not done with other tv success stories like Law and order,svu statistically one of the most successful shows on tv with over 11m viewers in america and over 200m world wide i can think of many other shows that have earned a shot at the big screen , CSI for example. so why do producers not want to try there luck at the box office??