The college teams back then were not playing NFL style football. So it took time to coach a QB into playing NFL football.
Most of the QB's that fail in today's game fail because they went to bad teams that had more problems then one man could fix. Which is why Bill Parcells team building mechanics still work today. Build a defense first, find a QB second. There are only a hand full of teams in today's NFL that emphasize defense first.
So you say all of those QB's were bad? True, but how was Dallas' defense back then?
Defense wins Championships! A franchise QB without a decent defense, will look average.