This topic really caught me by surprise and is an interesting point.
I think another part of the difference is simply how Tolkien intended his work to be read and thought of and how Disney chose to narrate stories.
We are all rather familiar with how Tolkien wished his works to be thought and viewed of more as legend, as accounts of great deeds, trials, etc. that could be seen as happening in a far off age where specifics don't matter as much as messages and eternal themes do. Stories that survive for quite a long time specifically because what they tell and their core values.
Disney pulls a lot of its source material from fairy tales, most of its older works from Grim tales. Which, if anyone's read the original stories you know they are quite different from that of Disney's interpretation. I think what Tolkien might have seen from this is similar to what others have pointed out, the almost excess of humor at the sake of message until all of these tales meld into one.
Before anyone goes on a witch hunt saying that Tolkien does indeed have humor and songs in his work so he is only being a snob of comedy towards Disney, he doesn't do so at the sake of his tales' core themes. There is little difficulty for a child to read The Hobbit and both enjoy the humor and become enamored by the struggle of the characters, or the message.
If anything it's a balancing act of knowing that your work is for children, but like many very old traditional tales (in the way of legends) which usually you would tell a child because of the advice/message they carry.
Maybe looking at how Disney treated Grim's original works, Tolkien felt quite hesitant to hand over or entertain the idea of giving them access to his books.
... Which after seeing those sketches, I'd be saying a firm 'Heck no!' too.
