The problem is by taking those laptops Snowden moved from being a whistleblower (a status that can be protected by laws and at least gives him a case in court) to an explict lawbreaker in the fact that US has strict export control laws governing protected data. Now, by means of either niavaty, cowardance, or ignorance, Snowden cannot avoid jail time in the US even assuming he isn't procecuted for leaking the information to The Guardian. Publicly leaking the information while going on trial in the US would have lead to people actually talking about the leaked information instead of focusing on the man as well as giving him a legal platform to stand on. I can't speak for Snowden's motivations be it cowardice, stupidity, or some disillusionment, but by leaving the country he hamstrung his own legitimacy and made it very difficult for the relevant information to be acted upon or even fully exposed.