I suspect it will be to get a balance of performance, both starting and stopping, emissions and of course mpg. From the bus and beyond video they suggested they were getting about 28mpg and drives well.
I personally have a view that a vehicle that large should only have as much power than it needs.
DSG tends to be smoother gear changes but can be jerky at low speeds. I suspect a bigger vehicle you don't want the potentially jerky performance of the DSG at low speeds and with a standard automatic you can just let the vehicle crawl more easily but with a slightly less smooth change at speed? Only a guess though.