The wide platform of a cat allows for an open "slot" between the genneher and the mainsail. This accelerates the wind over the back of the mainsail and creates more lift. A mono simply is not wide enough to properly lead a headsail in order to achieve this effect.
If this was true, certainly they would come up with poles to move their sheeting points further out (like a jib pole on a monoslug). In my opinion, I think it's more relative to the balance point between available righting moment, overall weight, and sail area to make the best VMG downwind. Cats have a lot of righting moment and can handle hot angles downwind. As such, our sails are optimized for these hot angles. Skiffs have a little less righting leverage so they run fuller bigger kites.
