I think that one reason is often that a film is way longer than a single TV series episode, so it requires a bigger "commitment". I can potentially watch 3 epis

I think that one reason is often that a film is way longer than a single TV series episode, so it requires a bigger "commitment". I can potentially watch 3 episodes of different TV series in a single evening or either 3 episodes of the same series, while I can only watch a single movie due to its length. Also, often TV series have more time to properly introduce and develop storylines, while films have to either simplify the plot or be unbearably long in order for the format to be feasible. This makes them often harder to follow or less interesting, for instance Game of Thrones would have been probably turned into utter rubbish if it was made into a film instead of (lots of) episodes - not that it ended that well anyway, though.