Ummm, laziness I guess! We just didn't want to go through the hassle of renting a car, having to find a parking space, driving on the other side of the road, not knowing where we're going, returning the car, etc. Plus, we would probably like to get a drink or two without worrying! Not trying to discourage you, it really sounds like fun to cruise around the island in your own vehicle, we just thought about it and decided to let someone else do the driving.
DH is a big Jeep fan, and we've decided to do the tropical jeep safari in Cozumel. It sounds like a blast! That might be an option to consider if you decide against renting one in Grand Cayman.
Whatever you decide, I'm sure you'll have a great time!