Walt's life was simply too eventful for a biopic. Take a look at The Aviator. It only covers about 20 years in the life of Howard Hughes, and does not even cover the the more famous stuff, like the tissue boxes and bottles of urine. For a movie to be made about Walt, many, many things would have to be left out of the movie, or it would have to be an Aviator-like movie where it just concentrates on a few years of his life, and where the audience knows what happens after the film is over.
A film about his early years growing up, going to Europe, working in Kansas City, and failing in his first studio might make a very good film, as we all know how Walt turned out. But studios today are much to fearful of audience reaction, especially to a film about an American icon. Imagine the response just here on these boards if they cut corners in the film or changed some of the facts. And unless Disney made the picture, it would be much to hard to clear all the rights issues.
The only biopics that seem to work in today's tepid Hollywood are of troubled singers, as they all seem to include a rise, a fall, and a happy ending. Ray, Walk the Line, even going all the way back to Coal Miner's Daughter, all of these films are pleasing to the public because of their universal stories and the music present. Serious biopics, like Kinsey and Capote were excellent films, but did not make any serious money. And the costs associated with a Walt film would be very prohibitive, considering non-musical biopics tend to not make a lot of money (The Aviator did not fare well at the box office). The only time serious biopics were made of famous people were the 1930s, when a whole load of them were made on everyone from Abraham Lincoln to Thomas Edison. Studios just don't like to make many anymore.
I'll end my rant by saying that Walt may be too big for a movie. His life is so famous that everyone would be criticizing something. Perhaps a mini-series would work, but Walt right now is one of those people, like George Washington, who will never have a biopic made of them, as they are simply too big for the screen. (A mini-series, however, is anyone's guess.)
Maybe in 50 years someone will do to Walt what they did to Mozart. Realize the factual life is not suited to a film, and just invent a story based on gossip and hearsay. And if it's anything like Amadeus, it should be a great film. Just as long as it's not a Mommie Dearest...