I can think of a few. For one, I don't wear contacts above water. I didn't want to pay the $150 fee the optician wanted to fit me for contacts just to get some for diving. Also some divers have to remove their masks frequently (think OW instructors) and don't want to loose contacts. Come to think of it, I wouldn't want saltwater under a contact.
I know it's not directly addressing your question, but is there any reason not just to wear contacts? I've worn them diving for years without any problem.