For me it's that the scale of things has gotten to big. Originally Superman or Batman dealt with threats to individuals. A criminal trying to fix a football game by trying to take out a star player or a corrupt mine owner whose workers were in danger due to unsafe conditions. Then in the Silver Age you got some large scale threats mixed with smaller ones. This issue the Flash is stopping a guy with an ice gun from robbing a bank and next issue he is stopping an invasion from another dimension, But even those large scale threats tended to be single hero things- Flash destroys the dimensional gateway before those invading aliens would even get on Superman or Hal Jordan's radar. By the Bronze age we had even lass of the large scale foes and most times the threats at most were to the hero's hometown. Maybe two Green Lanterns teamed up to solve a problem, but something that needed the whole assembled Corps plus the Guardians to personally get involved maybe three times a decade.
But now every story is on such a large scale that Superman's foes are a threat to the whole planet and you would wonder just why the Green lantern Corp isn't sending someone to deal with it. And the JLA is involved in cases that make it apparent that Earth is the ONLY place with heroes that can prevent universal destruction. The people of Alpha-Andromeda Ketrus III certainly don't have any heroes to send when the Anti-Monitor is loose or a rogue New God is threatening to undo the Big Bang. If all creation is imperiled then no one in the universe is more suited to deal with it than a pair of egocentric billionaires (Lex and Bruce). And after 14 billion years of relative peace these threats all appeared in the last 10 years because they were waiting for Earth to create the Speed Force, have a home grown set of Lanterns, and above all else give us a nut in a bat-suit.
This is all so much better than stories about saving regular individual people we could relate to.