It was, of all things, a result of the post-Crisis reimagining. Prior to that, both Batman and Superman were more "real" than their civilian alter-egos; Byrne and those who followed him flipped that around for the Man of Steel, who was once known to his close friends as Kal, not as Clark. Some of it was independent evolution of the character, some deliberate contrast to DC's other flagship character, setting up the "two sides of a coin" thing that finds its strongest expression in Superman/Batman.
As someone who prefers the DCAU version in almost all cases, I definitely prefer a Superman who's not a distant space-god, as well as a Bruce who's a bit cranky and very damaged but still has some humanity to him.