They were like only giving a flashback of the first season, and instead of bending the 4 elements bended the story.
They just took the best parts out like Aang taking shape of the water god in his Avatar state and Avatar Roku never appeared. And, they directed the movie straight to the end like it was such a drag for them to make this movie. It was just expectation as compared to the great series which till date holds an imdb rating of 9.2