The 70's were dominated by Seaver, Carlton, Hunter,Palmer and Ryan.
The 90's were dominated by Maddux, Martinez, Clemens, Johnson and, er, Ryan.
Actually, I would wager that there were more no-hitters in the 90's than 70's. For some reason I just remember a lot of them in the 90's, in fact, in 90 or 91 there were two on the same day or consecutive days.