Because there aren't many mainstream wheels that fit the Porsche properly and TireRack only carries mainstream parts.
Wheel Enhancement is arguably the best source for Porsche wheels and their prices are better than TireRack. Check out their site then give them a call.
BTW, I have a set of Kinesis K18R's that will be available in about 2 weeks if you're interested
They are in great shape. $2500 + shipping (sorry for the plug, they will go up in the classified section once they are off the car and pictures are taken.)
Here's a picture of them on the car.