Quick Google suggests they were convicted for espionage (under the Espionage Act), not treason. Though it's really unclear to me why espionage against your own country and for another would not qualify as treason.

edit: Sibling commenter had a view on it: "Technically, this wouldn't be treason according to the US definition, because treason is only possible in the context of a declared enemy of the USA (i.e., war)."

So part of the reason is that since the end of WWII we have not technically been "at war" because that requires Congressional approval and politics gets involved.