other factors wrote:
As others mentioned, running marathons had the novelty bump. Mountain biking (which many people do instead today) hadn't really started yet. Greg LeMond hadn't won the TdF yet, which created a cycling boom (later eclipsed by the Lance effect). There was no triathlon craze yet, and no sappy NBC coverage of the Ironman. And at that point, marathons were seen like ultras because they seemed so extreme. Basically, in the 70s and 80s, running marathons was the rage and it's unlikely it will ever be that way again.
I echo someone else in that the old Soviet Union helped to make track and field popular. Beating the Commies hooked people and I suspect they then developed interest in specific athletes (Americans were competitive from the 100m to the marathon remember) which led to interest in other events.
I would say that running marathons these days is pretty popular. I venture to guess that there are 1) more marathons organized in 2008 than in any year in either the 70s or 80s (how about any 2 years) and 2) there are more people running marathons than in any year during that era.
However, little to none of that seems to cross over into people being fans of track and field.