I think it's probably because the north side is more accessible to Centennial Parkway and the QEW, especially for GO buses. Plus CN may have wanted it that way, to keep their mainline straighter.
It may seem overbuilt now, but if more parking is added to the south side it won't be. And I doubt escalators will be installed -- they've been removed from older GO stations, probably because they become a maintenance nightmare during the winter with all the salt put down on platforms. The elevators will provide service for those with accessibility needs.
If the tracks were in a trench like at West Harbour we'd see much more being built. But this more "conventional" layout will be similar to other stations with tracks at grade (like Burlington and Appleby).