There are a couple of reasons for that:
-One is that, contrary to the occasional thought experiment, there isn't a magical Budd Fairy around to drop ready rolling stock on a line. At a bare minimum you're looking at a few days to move cars around, if not a few weeks (or more) to fix cars up. To be fair, IP might be able to move equipment from Saratoga in a few weeks. All of this is not free.
-A second is that IP isn't selling dome space yet. I have no doubt that they'll start ASAP, but until they do that likely means that they have extra capacity within the present set.
-Finally, the biggest concern is that IP isn't likely to want to spend a bunch of money moving a third coach into position and adding it only to find out that the sold out Hoosiers were an opening week fluke. They've had the route for less than two weeks, after all.