A fair section of Tolkien's essay "On Fairy-Stories" is devoted to this question. His position is that it was unfortunate that literature and drama were so often studied together. The two lend themselves to very different kinds of stories, and in a language that gave us Shakespeare, the balance was towards stories that work as plays.

Now many of his specific examples reflected technical limitations that movies have now surpassed. However much of what he said still applies in spades. For instance it is still true that, Very little about trees as trees can be gotten into a play. Hollywood finds it easier to produce fantastic effects than they do to surpass the limit that, In painting, for instance, the visible presentation of the fantastic image is technically too easy; the hand tends to outrun the mind, even to overthrow it. Their task is not made any easier by the fact that different minds have different limits. (See discussion here not that long ago about how many people find it impossible to follow any plot lines that involve time travel.)

As a final example I would ask how the following defence of fantasy that appeared in one of his letters could be translated to the screen. His writings are full of gems like this...

Dear Sir, Although now long estranged,
Man is not wholly lost nor wholly changed.
Dis-graced he may be, yet is not de-throned,
and keeps the rags of lordship once he owned:
Man, Sub-creator, the refracted Light
through whom is splintered from a single White
to many huges, and endlessly combined
in living shapes that move from mind to mind.
Though all the crannies of the world we filled
with Elves and Goblins, though we dared to build
Gods and their houses out of dark and light,
and sowed the seed of dragons -- 'twas our right
(used or misused)). That right has not decayed:
we make still by the law in which we're made.

Cheers,
Ben