Here’s a great clip from MTV where Peter Jackson explains the reasons for making The Hobbit into 2 films instead of doing that “bridge” film connecting The Hobbit to The
Lord of the Rings.
Really what it comes down to is that much of the bridge stuff will be grafted into The Hobbit (in addition to all the sequences they want to include), making it a longer
film that will demand more screen time as one cohesive story. Perhaps the most telling quote is this:
“There’s a lot material that we can retrospectively put into The Hobbit that doesn’t actually exist in the novel which is fun–I mean this is fun stuff because it sort of expands The Hobbit in a way that makes it very much related to The Lord of the Rings.”