Hollywood is scared of taking chances. Pulling ideas from older, successful material seems like a safer bet, but it's never a guarantee of success - you'd think they'd know this by now, but nope, the lemmings are still running to the cliff.
One example of a popular TV show turned into a flop movie: Mod Squad. I'm sure it didn't help that the show hasn't been regularly aired in most places since the mid-1970s, despite being a moderately successful show while it was on the air. The bulk of theaters are filled with kids in their late teens and early twenties, so the show was pretty much off the air sometime before they were born, or at least since they were old enough to understand dramatic TV. It never had the cachet and marketing/merchandising power of other shows like Charlie's Angels, Dukes of Hazzard, the A-Team, etc.
Another flop was Bewitched. The show was cornball when it aired in the first place. Also, NEVER put Will Farrell in the role of the straight man/the foil - his acting is about as bent as it gets. And it had some of Mod Squad's disadvantages - never had a big marketing/merchandising campaign behind it when it aired so you'd be hard-pressed to find a Bewitched lunch box or action figure/doll, and the show was pretty old by the time the movie came out so it didn't connect with a younger audience.
I could probably think of more if I worked on it. But I'm tired.
Simple fact is, if there's a moderately-to-wildly successful story in any medium behind the script, be it TV, Broadway, novels, the toy store shelves, what have you, and if it can be made on a moderate-to-cheap budget, someone somewhere in Hollywood is probably pushing to get a movie project greenlighted about it...