The allies and the Soviets had made a written agreement in November 1945 that three air-lanes would remain open. Unlike the road and train lines, for which no agreement had ever been formally made, closing these would have been reneging on a written agreement.

Stalin saw this as an impossible method for the allies and it would be far to unsustainable in economic terms to continuously supply Berlin with supplies. Stalin also didn't want to start another War. The USSR was still devastated from WW2 so they only pushed so far in terms of action.