Why aren't there more film series filmed back-to-back?

I know The Hobbit was also shot back-to-back, too. To answer your question, I can only speculate that committing to a back-to-back film production requires a tremendous amount of foresight and faith in the film project if it should succeed at the box office. If the first film is a success, they know there's a market for them to produce the sequels. Not every film is going to be a guaranteed success. Also, as you stated, it comes with a financial risk, and I also think the availabilities of the actors comes into play. Sometimes, they are already committed to shooting another movie by the time they are ready to begin shooting a tentpole film. However, it's Hollywood and they can work around it.