The problem there is that both movies were not very successful and, while they did make money, they didn't make enough to not be considered a dissapointment by Disney. There's also the followup show that, while it definitely has its fans, did not attract many viewers when it was airing at all. All of them have only reached widespread viewership and fandom over time. Couple that with the fact that a movie in this world would necessarily have to be expensive (much like Avatar it wouldn't really fit in the series if new movies didn't push new technology forward) a new movie is a massive risk when there is no guarantee of big returns.