Because Disney markets Snow White and all of the other known characters. They're on merchandise EVERYWHERE. Go to Wal-Mart or Target and go to the children's clothing department. I guarantee you there will be clothing with the Disney Princesses and Pixar's CARS. There is a lot more exposure to the "main" Disney characters from society. Parents can share any movie with their kids, yes, but I'd be willing to bet my bottom dollar that more children 6-12 in the U.S. have seen Disney's SNOW WHITE AND THE SEVEN DWARFS than WHO FRAMED ROGER RABBIT?
When a child goes to the Disney theme parks, they can name almost any character by name. I doubt they know who Roger Rabbit is.
(Roger Rabbit isn't the only less known character in the parks. They still have Brer Rabbit, Brer Fox, and Brer Bear from SONG OF THE SOUTH in rotation. Etc.)
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle