I have a couple of theories to explain as to why those last six months on NBC and the syndicated run were the only episodes USA aired.
My first theory: In 1991, USA began making deals with Reg Grundy Productions, starting with reruns of Scrabble. Reruns were a ratings success, so USA decided to pursue $ale a year later. USA chose to lease for roughly 130 (give or take a bit) from the last six months, and the syndicated run. But ratings were not as successful as Scrabble, and were only on for 2 years, compared to Scrabble which was on for 4 years, giving USA plenty of time to air almost all of that show.
My second theory: Those last six months and the syndicated may not be all that exists, as Mitt Dawson claimed, but rather the only ones Reg Grundy America (now part of Fremantlemedia North America) had the rights to, and all the others from the 1980s NBC run are owned by Fremantlemedia Australia (which Grundy Television is a part of). But then how did GSN get Who Dares Wins a decade ago?