Why are pictures not allowed in museums?

Question:

Why are pictures not allowed in museums?

I was at the Museum of Modern Art in NYC yesterday and photography was allowed at some displays and disallowed at others. Other times, photography was allowed but with no flash.

Best Answer - Chosen by Asker:

They will tell you that the high-intensity of a flash will damage the art.

However, it's because the museum *sells* prints, so they don't want you taking photos and printing your own.