Phil, I agree with you that the further north you go the better your chances of catching a big DD bass. Although I caught an 11.4 bass at Loxahatchee in the late 80's, and a DD at Okeechobee, and at a road pond along I 75, I chalk that up to fishing 300 days a year and just being in the right spot at the right time.
There is a good reason for the lack of DD down here. An old timer told me 50 years ago, every years our waters flood, big bass move out into the miles of shallow grass to feast on everything that lives there. Then we always have the dry season, and a high percentage of big bass get caught up in the shallows and don't make it back to deeper water. They become Bird, Turtle, and Gator food. This culls the population and keeps them from reaching there max size. Although some do make it in time, many do not. The Everglades are unique with tens of thousands of square miles of shallow grasses with only a small percentage of deep sanctuaries, the man made canals, and gator holes! I never forgot this insightful conclusion from a guy that grew up on a ranch in Eastern Naples, hunting and fishing. The glades just lose big fish every year during the drought. Still the numbers of normal fish caught are just off the charts.