I asked one of my contacts at Tesla about this last month, and he was a little cagey about it, but basically said "we're plum sold out".
I think there's more to it, like manipulating the number of inventory cars available to push people to order new/custom cars.
In any case, if you're looking for a new/inventory car, call your local Tesla store/showroom since they probably have access to lots of cars that are not listed online.