Frodo & Sam had a much more pivotal role to play, for half the journey they were own their own, while Bilbo was just kinda along for the ride, even though he did prove useful and even took initiative as the journey went on. It's not unlike the book, they just expanded other characters and rightly so.
Funny, how some people have the opposite reaction to myself. Bilbo had more to do and proved more useful as the journey went on. Other than the parts in the Shire and perhaps the meeting with Gollum, Bilbo wasn't much of a focus in AUJ.