There is a huge difference between Crete and Malta. For one Malta's defenses were far greater. That means you can't just have an airborne invasion. If you are coming by sea the RN is going to be a real threat. As it is look what happened to the seaborne leg of the Crete invasion. Then look at how long allied fleets had to stay around and provide support for the various invasions in the Pacific. The axis simply could not afford to have their surface forces pinned like that. They also lacked a lot of the special equipment the allies had as well as much in the way of experience at sea based invasions. Even if they succeeded the loss in shipping could well mean they couldn't hold it. I'd guess they had a lot less than a 10% chance of success and failure means the loss of most of the invading force.