Because the Carrera GT doesn't comply with the US safety regulations as of January 2006
Porsche has decided to scrap 250 of the 1500 lowering the total number to 1250.
396 Carrera GT's have already found a way to their owners in the US and the current owners are not complaining. Most of the US buyers buy a car like this as an invesment, less produced means more money. 1250 worldwide is still a huge number, other producers like MB and
Ferrari only build 300 to 400 exclusive models to be sold worldwide.
The reason why the GT doesn't comply with the 2006 US safety regulations is because as of January 2006 every car sold has to have 2 stage airbags. According to Porsche it's not profitable to make such an adjustment on a exclusive model so they're scrapping the final models to be sold in the US.