[eesmith]

In some sense that's tautological. We know some publications exist that others thought were great works, but those have been lost, so we cannot now say they are great works. https://en.wikipedia.org/wiki/Lost_literary_work

> for some reason

Had the fire at Ashburnham House burned the sole copy of the Nowell Codex, we would not now have Beowulf.

Had Max Brod followed Kafka wishes, we would not now have The Trial.

So the "reason" can be happenstance, and not due to how we currently regard its place in literature.

The question then is, what other works might we now regard as "great", but for happenstance. Some may even still be available, and simply unread for centuries, waiting to be rediscovered.