The Disney Transportation System is severely taxed. Monorails that are bordering on disrepair, a bus system that is so overcrowded during peak times that Disney must outsource to third party operators to increase their fleet. Not to mention that reliability, while good, still not magical, especially if one of said transports suffers a mechanical issue. The boats are slow, crowded, and have their own issues.
I don't think Disney's goal here is "hey let's get more people on Disney Transportation". As for keeping people onsite, sure that's a goal, but for $20 bucks I don't think many people are going to give up the freedom of taking a ride to the outlet malls, Uni, SeaWorld, or I-Drive attractions and restaurants.
If the goal was to get rid of cars and move people to DT, Disney wouldn't have spent a boatload of money building parking garages, widening Buena Vista Drive, and constructing a new off ramp for I-4.