I think also because one of the elements that made the originally so engaging in two of the three films is that a major plot element, and having to come up with this elabororate and perfectly timed plan that allows a set of circumstances to return.
This was an exciting development which served as the climax for the first and third films. The whole story built towards it. Being able to fix the past before the time came where you had
to try and go back to the present. Without it, there isnt all that much at stake or sense of urgency.
Your right Marty is the main character and I'm not saying get rid of him or anything. But it might be interesting to be able to just play Doc for one level or something. Doc is a pretty main character as well after all.
Ok, Doc for a level might work. Jules and Verne for any amount of time, no. No, no, no, no, and no.
Back to the Future is a buddy film about Doc and Marty. Adding anyone else into the mix just doesn't work. Why do you think they got rid of Jennefer so quickly in the second film? Becuase the writers knew she had to be out of the picture to keep the Doc and Marty dynamic in the forefront.