i imagine the reverse situation, the soviets traveling into the west relatively uncontrolled had the capital been on the west side. Seems very precarious. It's not like they're being escorted for the entire duration of the journey. How could they agree to such a scenario?
In 1948, the Soviets began a blockade to isolate West Berlin. The idea was that the West would be forced to surrender West Berlin and the Soviets would get the entire capital. The British and Americans responded with the Berlin airlift, which delivered supplies to West Berlin by air. Once it became clear that West Berlin could be supplied indefinitely by air, the Soviets were forced to acknowledge that they couldn't gain West Berlin without a war that no one wanted and so they dropped the blockade.