Probably best to ask on the Villa fan's forum. There was a perception (at least in my mind) that he was not obvious manager material. More of a coach.
Coming from a family of Villa fans I remember that period well. Barton was seen as Doug Ellis's man at a time when most fans really didnt like Ellis. He (Ellis) came after Saunders who was seen as belonging to the previous preferred boardroom regime. There was a real hostility towards Ellis, they blamed him for the clubs demise post '82.
I think Barton came to Northampton rather than a bigger club because his health problems were already known and limiting.