Why did AMB Karl Eikenberry's classified cables on his views of the capability and capacities of the Karzai Government leak to the media? 

Internal Executive branch documents like Eikenberry's assessment and McChrystal's report on Afghanistan tend to be leaked as consequence of manning a typical Administration with three types of people, each of whom wants to derail decisions they see as trending the wrong way, and see leaking as a path, though not necessarily a virtuous one, to gaining the upper hand in a debate:

(1) The True Believer is someone who cares desperately about an administration's success and believes the effects of a policy decision by POTUS may bring about catastrophic consequences overseas but, more importantly, will bring a catastrophe to the domestic political fortunes of the Administration.  The True Believer normally worked on as a mid-level staffer on the campaign and comes into the Administration often with great academic credentials, or a good political Godfather from working on the Hill, but precious little practical experience in national security affairs;

(2) The Long-Serving Bureaucrat is someone with extensive hands-on experience in national security policy formation, who has watched Adminstrations come and go, and believes their point of view isn't getting a fair hearing often because Administration politicos are paying more attention to the Hill than they are to the global security environment.  The Bureaucrat would rather opt for establishing and maintaining the credibility and legitimacy of a standardized national security policy review process but they also know how to adapt to and overcome political speedbumps; or

(3) The Professional Politician is someone who wants to force the administration to publicly defend its choices.  The Professional Politician may have some experience in national security affairs and some understanding of foreign policy issues but those experiences are far outweighed by the politician's sense of how an issue will play in the media and on the Hill.  The politician also casts an eye-towards how the leak will improve their standing in an Administration as the debate evolves after the leak. 

No matter which of these types of people provided the media the Eikenberry and McChrystal documents, the POTUS and the National Security Advisor should take the actions as a warning that they are not building consensus within the government, either for their process or their preferred course of action.