A few factors made a contribution to these swings, including adverse supply shocks in the 1970s, and favourable ones for much for the 1990s. These sorts of things can make monetary policy’s job harder or easier, though of course monetary policy ultimately has to take responsibility for price level outcomes in the long run.
One of the central lessons drawn from that experience was the importance of expectations in the inflation process. The more ambitious expansionary policies could only deliver the intended gains while expectations remained stable. Once expectations started to move up, policies were less effective in boosting output, because people behave differently when inflation is thought to be a part of the economic furniture.