Why might the earliest music of the Middle Ages have been passed from generation to generation largely by ear?

Answer ... because the printing press has not been invented.

Explanation:

Note that, the time that is been referred to as the middle ages is between 1000-1500 AD. The time between the fall of imperial Rome and the beginning of the early modern Europe.

During this time, there has never been any invention for writing or typing the lyrics of a song, so that it can be kept for future reference. Therefore, the only way of grabbing the lyrics of a song is by singing from a generation to a generation.

At this time also, there is no formal education to teach people how to write.