Why did Bill Gates could not ride his imported Porsche 959?

Car Facts
Bill Gates imported a Porsche 959 supercar before it was road legal in USA, and the $470,000 car was kept impounded for 13 years till the law was changed.
Why did Bill Gates could not ride his imported Porsche 959?
Bill Gates imported a Porsche 959 supercar before it was road legal in USA, and the $470,000 car was kept impounded for 13 years till the law was changed.