Assange _was_, in fact, charged under your Espionage Act, I'm afraid.
Your Constitution contains America's very specific legal definition of treason. Baking that into the Constitution was a very prudent move by your Founders who had seen their previous government make every form of dissent and whistleblowing into treason.
Julian Assange, while not an American citizen and not formally classified as a traitor, has been accused of crimes related to the dissemination of classified information. In the United States, the distribution of classified information can be viewed as a threat to national security, but this does not equate to being classified as a traitor under U.S. law. The term "traitor" typically refers to actions that directly harm the state or aid an enemy power, and Assange's situation is not categorized as such.
