There was, of course, also the factor that Fairchild were at risk of going bust in the days before companies in bother were simply hoovered up by Boeing, Lockheed or Northrop/Northrop Grumman. Or BAE...
Also, the A-7D vs YA-10 fly off didn't produce quite the results the A-10's proponents claim, namely complete superiority for the A-10. The latter was better in certain scenarios - very low level, gun runs on tanks, but many of the issues with the aircraft which have been discussed on Arrse before were hinted at in the trials - the pilots who flew them (with operational experience of CAS using F-100s and F-4s in Vietnam) concluded that there was no clear winner and that the two aircraft complemented one another.
It was also noted that the A-7, which had an autopilot and excellent weapons-delivery systems for the time was better suited to operating in the lousy weather likely to be found in Northern Europe.
Indeed, some cynics suggested that the fly-off was designed to see whether the A-7 could fly like an A-10, rather than which of the two airframes met the requirement. When the answer was, in effect, 'both are useful' the USAF concluded that buying the new aeroplane (which was what Congress wanted them to do) and handing the A-7s over to the ANG to replace the older attack aircraft which were in use (the F-100 and a squadron of F-105Ds). The size of Aerospace Defense Command was also drawn down in the mid-1970s, and the ANG F-102 and F-104 (Puerto Rico ANG) squadrons were re-roled with TAC using the A-7.
Again, a cynical interpretation is that had the USAF decided that the A-7 was the better bet, there'd have been a political storm, the Army would've complained about not taking CAS seriously, and - worst of all - the ANG would've been getting brand new aeroplanes while USAF units flew older aircraft (which, horror of horrors, were derived from a USN aeroplane). The desire to have a mixed fleet of A-7s and A-10s was suppressed and the A-7s sent off to the Guard.