A resposta é bem chata: Tolkien não gostou da ideia.
Peter Jackson, que é diretamente uma adaptação cinematográfica um pouco mais conhecida, conheceu Paul McCartney no Oscar em 2002, onde aprendeu sobre esses planos. Ele deu um comentário a um jornal neozelandês :
Jackson, whose own version of the first book in the fantasy trilogy, The Fellowship of the Ring won four Oscars this week, told the newspaper that the Beatles plan fell flat when author J.R.R. Tolkien rejected the plan. […]
“[The film] was something John was driving and J.R.R. Tolkien still had the film rights at that stage but he didn't like the idea of the Beatles doing it. So he killed it,” Jackson told the newspaper.
Matthew Schmitz tem um artigo em First Things escreveu um pouco mais sobre os planos, e tem um palpite plausível sobre por que Tolkien sentiu animosidade em relação aos Beatles:
In a 1964 letter to Christopher Bretherton, Tolkien complained about “radio, tele, dogs, scooters, buzzbikes, and cars of all sizes but the smallest” making noise “from early morn to about 2 a.m.”
“In addition,” Tolkien wrote, “in a house three doors away dwells a member of a group of young men who are evidently aiming to turn themselves into a Beatle Group. On days when it falls to his turn to have a practice session the noise is indescribable.”