Why do so many people think that Hauptmann was innocent?
Because having an innocent man to railroaded to his death by a bunch of stupid and corrupt cops is a more interesting story than the police working hard to catch the right man. A lot of people are fascinated, and comforted by, the idea of police misconduct leading to injustice. In an era where what one believes is more important than what one knows, Bruno Hauptmann can be innocent, and the Lindbergh baby alive, selling computers in Connecticut.