Bro, channels don't want us to watch movies. They want us to watch ads. They will air a movie only if all ad slots in the movie have been filled (especially during premium times like noon and evening).
Yesterday i was reading about tv broadcast of movies...it said there is similarity between theatres and tv broadcasting: theatres show movies to sell refreshments like popcorn and cola....tv channels show movies to sell ad slots.
So channel will wait till all ad slots have been bought by companies. Then they will air the movie. Older movies, no company will want to place ads. They will want to put ads in newer movies.
Profit from selling of ad slots will be shared by the channel and producer of movie.
Why channels renew some movies? That's because they are getting good profit from the movie. Otherwise, they will return the movie rights. Then producer can give rights to some other channel.