Porsche would not allow it to be crash tested, so it wasn't street legal until 1999:
"The 959 was not street legal in the United States prior to 1999 when the "Show and Display" law was passed, although an unknown number were imported via the "grey market" during the late 1980s as show pieces. During the model's development, Porsche refused to provide the United States Department of Transportation with the four 959's they required for crash testing, and the car was never certified by the NHTSA for street use in the U.S. With the passage of "Show and Display" the crash test requirements were removed and importation of the 959 was allowed, assuming the car could meet the applicable emissions standards that existed as of 1987. The 959 can be fitted with a catalytic converter and a rechipped computer which allows it to meet those emissions requirements. However, most owners refuse to modify their 959s, and the cars remain collection pieces. Most 959s are currently in the hands of collectors, but the ones that are available do occasionally come to market, with prices in the region of 180,000 - 250,000 EUR (cars produced in 1987/1988). It is impossible to estimate the price of cars from the highly limited batch of 1992/1993."
Bill Gates bought two when they first came out and they were impounded, no one knows what happened to them. Bruce Canepa also imported quite a few to the US. I believe Jerry Seinfeld has one.
Bill Gates still has a least one 959, which thanks to some lobbying, is drivable to a limited extent (I think he can only do a couple thousand miles a year in it).
God i love this car, it was the first Porsche i've ever had seen in my life when i was 11 years old, which has gotten me to fall in love with Porsche. I remember that day like it's yesterday!
it wasn't an attempt at a supercar it was a homologation special for Group B much like the GT1 (the 911 GT1 was homologated for Le Mans GT1 class). only 337 were made total including all prototypes