Because the process for ordering one seems really complicated.
According to Porsche, you can buy a 918 Spyder from any Porsche dealer anywhere in the world, provided they've signed a participation agreement with Porsche. Details are few and far between on what that agreement entails. What Porsche is saying, though, is how dealers who sign the agreement will order their cars.
Dealers who want to place an order for a 918 will have to submit an "Allocation Request Form." Porsche will reply with an "Allocation Response Form," which tells the dealer whether there's a car available for them. Once that's done, the dealer submits an order form, along with a $200,000 down payment. Porsche will respond with a confirmation and an estimated month for production.
The next step is another $200,000 deposit 12 months before the production date. The balance – $445,000 plus shipping, if you're counting – is due when Porsche says the car is done. Porsche says the car will not be shipped until it has been paid for in full. The reason, apparently, is that it won't be building any of the hyper-expensive 918s on speculation, and that each car needs to have an owner before it is built.