How come when they Adapt a TV show into a movie they don't stand by the formula of the orignal Show?!?

tv shws are made to work at a specific length so streching the running time means padding out a story to fit the time .it's not just adaptations the original coloumbo works better that the revival because the films differed inlength .while all the new ones are padded out to 90 minutes. the reason most tv adaptations don't work for me is that i like a series ie starsky and hutch ,lost in space and such because i like the way the actors play the parts and to cast different actors just ruins it for me.the worst for me was mission impossible.