Why is there so little time to create a tv episode?
I think this comes down to the fact that a TV series has a lot more footage than a movie does. A movie usually lasts between 1 hr and 2 hrs, so there's only that much footage to work with. But a TV series, even if it only succeeds in getting 20 episodes, would have a lot more.
A 20-episode half-hour series would contain approximately 10 hours of footage. And because companies have to get a lot of footage available so they can air several episodes within a week or so span, to accomplish this, they would have to spend less time on the quality and work more on the quantity. This effects the animation.
If the LBT series were to have footage equal in quality as the movies, then the time taken on each episode would have to expand. Instead of getting one new episode every week, we could see a difference of perhaps several weeks to a few months for a new episode. However this judgement I'm making is only based off the fact that movies, with better quality, have less footage to work with while TV series, with lower quality, have a lot more footage to work with. So I could be wrong.
But that would be my answer as to why the animation quality in the TV series is low compared to the movies. There is more footage to work with, a lot more, and they have to spend less time on it if they are to get a new episode in once every week or so.