Notice how many TV shows are made into films, but how many films are made into TV shows? Exactly: not many.
Out of curiosity, I'm going to try to name some. Buffy the Vampire Slayer, MASH, Lock Stock & Two Smoking Barrels, Star Wars, Stargate, Highlander, Planet of the Apes, and Le Femme Nikita all spring to mind. I don't think it's that uncommon for films to move to the small screen, though I agree it's definitely a case of 'downgrading'!