Marathon running participation is decreasing for several reasons.
1] The economy is better now. People are more able to focus their energies on more costly pursuits than running marathons.
2] It became passe to run marathons, as anyone could literally walk the event and a seven hour finisher has as much clout on social media reveling their accomplishment as a sub three hour finisher.
3] Millennials don't see the appeal of long distance running and prefer zombie chase, mud runs or doing yoga or spin classes.
4] Races registrations have reached unconscionable prices.
As an example, the Las Vegas Rock n Roll Marathon wants $179 for their full marathon, $165 for the half and $109 for the 10km. The full marathon now is little more than running up and down three or four streets in a serpentine pattern and doing circuitous laps in a parking lot.
5] Runners wear out or get broken in some manner over years of marathon running. I have probably run my last one and my speed is never coming back. But as I have over 20 finishes to my record at the distance, I can walk away satisified.