In order to portray a Toontown that realistically showed all of the characters that would have lived there, Disney had to negotiate to include outside characters. Due to various egos and rivalries among the studios, it was not an easy process.
Some rights holders saw that if this film was successful, it would introduce their out of the spotlight properties to a new generation. That's why Betty Boop got her scene stealing turn, voiced by Mae Questel, her original voice.
Warner Brothers, on the other hand, were not so easy to deal with. Their Looney Tunes characters had always played second fiddle to Disney. Bugs Bunny has never been as popular as Mickey Mouse, whose merchandise frequently outsells Warner Brothers' entire licensing portfolio.
Knowing this, Warner Brothers played hardball. They insisted that Bugs Bunny would only be permitted in the film if he had the exact same amount of screen time as Mickey Mouse. Like a petulant, jealous actress, Bugs Bunny's handlers would ensure that, at least in the world of Who Framed Roger Rabbit, Bugs would be equal to superstar Mickey Mouse.