I would guess several things:
Most people did not run a marathon prior to college graduation, so it presents a new challenge.
It really is a challenge. Whether to finish at all, to break a time barrier, to set a personal best, there are self-defined challenges for all the participants.
Dragging yourself to the track to do structured intervals and to endure that kind of pain loses its luster, so setting bests in the 5k and 10k could be difficult.
Running intervals alone takes a certain kind of person. Hard intervals are almost a right of passage, done with teammates and under the supervision of a coach. Running them alone can be very lonely.
(And yes, I know that fast marathon runners do intervals, but a lot of marathon runners don't).
You get more "likes" on social media for finishing a marathon than for a 5k or 10k.