Super-villains can only poison the water supply, create Red Kryptonite and hijack nuclear weapons so many times before that kind of thing gets old. Part of it is in response to changing culture, as in the nuclear example. Stories about fear are human stories and our fears change, often growing more complex to reflect a more complex world. And therein lies the biggest part of it. Our lives, whether more complex or not, feel more complicated at the end of the twentieth century than they did in the Golden Age.
Nothing can be plain and simple anymore.
Escapism values good versus evil in a battle for the fate of the world. That doesn't happen in the real world. The simplified universe of comics makes them approachable. So a balance has to be struck between the gritty reality that we crave, the varied, complex stories that draw diverse audiences in, and the iconic power of the four-color world. Most often, the issues I stop reading err too far to one extreme or the other.