Well the thing is they explained most of their childhood through the course of the series. And the 60s wouldnt be very interesting seeing as most of them were born in '59.

It would be cool to have more of them but it wouldn't work.....maybe the could make the 80s but not the 60s....

Actually, half of them were born in ‘60. Only Kelso and Hyde were born in ‘59.
Everybody else except Jackie (born ‘61) was born in ‘60.