The first problem with going after Assange is that the effort is likely to fail. Extraditing Assange from England (where he is now) or Sweden (where he may go to face charges of sexual assault) would not be easy, especially since Assange's actions might be deemed a "political offense," for which exceptions are made to extradition obligations.
Even if the U.S. government surmounts this hurdle, a criminal conviction is not assured. The most relevant law, the Espionage Act, is famously overbroad and thus an uncertain basis for prosecution. This is one reason the government has never successfully prosecuted a member of the media for soliciting or publishing classified information. Nor has the government ever successfully prosecuted a non-media organization for solicitation or receipt of classified information.
A failed attempt to prosecute Assange would be worse than not prosecuting him. It would make the United States look even more ineffectual than it does as a result of the leaks.
A successful prosecution, on the other hand, would not achieve the desired deterrent effect. WikiLeaks copycats are quickly proliferating around the globe, beyond the U.S. government's effective reach. A conviction would make a martyr of Assange, embolden copycat efforts and illustrate the limits of American law to stop them.
A conviction would also cause collateral damage to American media freedoms. . . .