I hate to sound cynical but the most likely reason for the photography ban, aside from ordinary standard human stupidity, is that the Sistine Chapel is basically a claustrophobic shoe-box that's been tarted up by an artist who grew so bored of his work that the final 30% is barely a colored-in sketch. Lauded as one of the great artistic works of Europe it's a huge disappointment, not to mention the Church's embarrassment of the fact there's a naughty (but wholly understandable) presentation of Julius II as one of Satan's malignant creatures.