Because they are dead. That's the sad and only answer I have. I am retired. I can research maps until the cows come home. Jon had a full-time, 24/7 demanding job. He was a landlord. He had friends and family all over the world to keep in touch with. He was a new dad. He had Ellen to keep happy, as husbands must. There was Covid AND the adjustment of living in isolation in a rural, remote community.
(I really like
@RedHaus and others' theories of Jon researching the mines. That seems like what you would do in your spare time. )
I will always believe, until proven otherwise, this was a spontaneous walk, ill-planned and risky, in the cloud of new parenthood and other distractions. That it was taken without fully understanding the area weather or trails. Perhaps in the way we tend to treat our own neighborhood more cavalierly than a strange place.