@JGoldman10,
Probably because Looney Tunes: Back in Action was a box office disappointment and Warner Bros didn't want to spend money on another possible flop.
Plus, the show came out around the time traditional animated films were failing at the box office while computer animated films (Shrek 2, Madagascar, Finding Nemo, Ice Age, etc) were successful.
Also, I don't recall Duck Dodgers being popular enough to justify a film