Basically because no such person ever existed. He was a compilation of two pilots whose names were leaked to the press, particuarly Jack Anderson, during the air war.
One was named Dien Ton (spelled TOONS in Vietnamese telegraphics, who had some rather amazing adventures but is only credited with a single victory which can be substantiated. The other was Dang Ngoc Ngu, who was one of the VPAF ace claimants and did most of the rest. However, both of these pilots flew MiG-21s and once converted they never went back to the MiG-17.
There was also a pilot named Nguyen Van Tho, who was a MiG-17 pilot and did fight with Cunningham/Driscoll on 10 May 1972. He may be the one, but since the VPAF insists he survived the combat (and from Cunningham's report the pilot they fought probably could not have) no one is sure to this day.