I'm assuming by "the QBs" you are referring to Steve Taylor and maybe McCathorn Clayton. Steve Taylor just flat out wasn't a good passer- the passing game during his career was check to see if the #1 option is wide open and if not, scramble. Taylor was a great scrambler, but he didn't match the hype he had coming in. McCathorn Clayton was an option quarterback. Period. Very good runner, poor passer.
The other QBs of the 80's- Gdowski, Gill, Sundberg, Quinn- who were above 50% passing and I remember them as very good passers. But their completion percentages still weren't great. Why? Well, Osborne and Solich understood that to be a good option team you had to put the time in on it. We didn't put as much time in on it- we recruited run blockers for linemen and run first QBs. And that system worked well. Second, it was a different era for football in general. More passes went down field and there were far fewer easy throws. Run based teams (especially NU) sent fewer receivers down field. Fewer screen passes. The Captain Checkdown QBs of today who are scared to throw down field wouldn't fare well in most offenses of the 80's.