there is the possiblity the the Sovs sent Oswald back to the USA and told him to wait for further instructions. On the assuption that Hoover would decide Oswald was a Soviet agent and commit resources to discovering his mission.
As Oswald had no mission the FBI would fail. As Hoover was paranoid, this would only result in committment of further resources.
Oswald grew bored and wrote a mission for himself.
now both the Soviets and LBJ have a motive to cover up any connection between Oswald and Soviet Intelligence.