It's probably because the Disney water parks are so out of the way compared to Aquatica and Wet n Wild. At those parks you could theoretically park there for free and then walk a couple blocks to Sea World or Universal. Where could free-parkers walk to from TL or BB?
From the Disney water parks you can get a bus to any resort, transferring there to reach the theme parks, just as easily as people do so from Downtown Disney. Blizzard Beach to Coronado Springs is even walkable, though its a bit of a hike, for more direct theme park access.