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Papers, stones, and most materials used in the past have some sort of acidic content. The acid can cause the papers or the artifact to self-destruct.

Oxidation is activated by both light and being exposed to the air

You will find such a prohibition in many museums. They have their reasons, and they don't need to tell anyone what the reasons are. It could be for the distractions, or for the damage from the light. And of course in light shows and at movies, flash would white out the display.

You're lucky they permit photography at all. If you go to the Rock and Roll Hall of Fame in Cleveland Ohio, you will have to give your camera to the coat check person.

Been there, done that.

The strong light can speed the deterioration of delicate exhibits, even behind glass.

it distracts other visitors.