Because they are snobby gatekeepers who think making art more accessible would dilute their value. These sorts are very common in the art and academia world.

Or because they are ignorant, scared and facing a technological disruption that might threaten the value of the original, which is the very reason the museum exists. You get a similar dynamic with over-enthusiastic corporate takedown notices.

I read it like this: the museum wanted to safely preserve the original so they made a digital copy and hid it in a drawer. The author pointed out this was not lawful, and the museum panicked because they intended it as a digital copy protected similarly to the original. So, commendations to the appeals court for identifying the key philosophical issue; regrets that they ignored the actual law.