I suppose the first Hobbit film (1967, featuring Princess Mika, and the only adaptation he could have seen in his lifetime) didn’t change his mind about cartoons.

It is basically a narrated slideshow completed just to fulfil the terms of a book rights contract. The producer had it cheaply and hastily drawn by one of my favorite artists/animators Adolf Born, known for his other work like Mach a Šebestová.

This article never does fully explain why, only taking the issue one step further:

Tolkien refers to Disney’s works as “vulgar.” Tolkien also believed that fairy tales had become hopelessly infantilized

It seems just a matter of personal distaste?