Back in the day, Decathlon was huge. Jenner became a huge celebrity after winning gold in 1976. Everyone in the US was watching the decathlon that year and Jenner's lap with the American flag started a tradition for winning athletes that stands today.
The event really faded in popularity as the US failed to be competitive in the event after Jenner retired until the Dan and Dave years. But by then, the East v. West rivalry of the Soviet Block v. NATO aligned countries had fizzled after the fall of the Soviet Union and reunification of Germany. So, when Dan won gold, it really wasn't a big deal. Sadly, when Ashton Eaton came along, the event was pretty much at a low point in popularity even though Eaton was a total beast and amazing to watch.
I do agree that the decathlon is hard to watch when you are seeing it side by side with the individual events. Any decathlete who is competitive with an individual hurdler, sprinter, etc. will just blow away the other competitors. And when there is a competitive heat, it still is a bit of a sh#t show compared to the individual event.