for the majority don't know how to maintain a turbo car. So if they released all turbo cars, there would be more of them in a junkyard because people would be changing the oil every 6000 miles, putting shitty gas in the tank and not keeping up with the maintanence. Nissan wouldn't look very good if all of their Z's started breaking down, thus the NA was perfect for the american market, lower maintanence, same looks, no turbo worries.