The obsession with marathons came about with the US Olympic Trials. Wait, hear me out. Anyone can qualify for the trials, its open to anyone who hits a time standard, whereas they sometimes/often take a descending order list for the track trials. 200 people could run the OT marathon when maybe 30 run the track trials. Everyone wants to be like the best, so it dropped down from there. Back when 2:22 got into the trials, guys with 2:30 potential moved to the marathon to try to get down to that, the 2:40 guys who wish they could beat them moved to the marathon. Soon all their friends and family members wanted to complete a marathon too. Only for them, it wasn't about pr'ing or running fast, they weren't willing to put in the work to train adequately, so it became about completing it, when the marathon was 40-60% of their average weekly mileage. Then it really took off. All the friends of the friends and family members of the friends wanted to complete one too. It became a poor man's Everest, one that was more easily accessible too. Enter tshirts, medals, bands on the course, advertisements all over the place.
This isn't a bad thing, but tv networks should capitalize on it. Insist marathons are held in prime time and broadcast those to huge audiences. Or maybe thats why its so popular. People view running as a fitness activity, not a sport.