The sad truth of the air/sea package is that the cruise line really doesn't stand behind it that much - so it really isn't much different than doing your own air as far as liability. I can tell you right now that Delta won't do anything. They got you to MIA and that is the end of it as far as they are concerned.
Weather holds are a strange thing. One can affect you even though it has nothing to do with your flights or your departure and arrival airports. I also think your flight times were too tight to begin with. Did you book directly with RCI or did you use an agent?
Why not a credit? Because contractually they don't have to. I hate to be blunt, but that is the bottom line. I assume this was your first cruise with RCI, or with any cruise line?