Here's the real story
The film is to some extent a self-serving whitewash produced by the Disney studios. The real story: The Disney musical adaptation was released in 1964. Primarily based on the first novel in what expanded into a series of eight books, it also lifted elements from the sequel Mary Poppins Comes Back. Although Travers was an adviser to the production, she disapproved of the dilution of the harsher aspects of Mary Poppins' character, felt ambivalent about the music, and so hated the use of animation that she ruled out any further adaptations of the later Mary Poppins novels.
At the film's star-studded première (to which she was not invited, but had to ask Walt Disney for permission to attend), she reportedly approached Disney and told him that the animated sequence had to go. Disney responded by walking away, saying as he did, "Pamela, the ship has sailed."
Enraged at what she considered shabby treatment at Disney's hands, Travers would never again agree to another Poppins/Disney adaptation, though Disney made several attempts to persuade her to change her mind. So fervent was Travers' dislike of the Disney adaptation and of the way she felt she had been treated during the production, that when producer Cameron Mackintosh approached her about the stage musical when she was into her 90s, she acquiesced on the condition that only English-born writers and no one from the film production were to be directly involved with creating the stage musical. This specifically excluded the Sherman Brothers from writing additional songs for the production. However, original songs and other aspects from the 1964 film were allowed to be incorporated into the production.
It is better to be kind than to be clever or good looking. -- Derek