Why is wicked complexity so difficult to manage? Well, to begin with, the systems giving rise to the problem cannot be definitively described or defined. Drawing boundaries around such systems is necessarily arbitrary; any effort to define the system thus oversimplifies other aspects of it, and downplays or ignores critical couplings between what you have defined and other, external factors.
Thus, for example, the current penchant for defining climate change as a primarily environmental issue, rather than as a complex and difficult social, economic, and cultural condition, is one reason why policy initiatives such as the Kyoto Protocol have failed. Defining cities as discrete entities that must be managed for particular outcomes (e.g. environmental quality), rather than viewing them as emergent phenomena characteristic of our species, with myriad dimensions and domains, is another obvious example of dysfunctional (and amazingly unintentional) oversimplification.