Answer:
Because West Berlin, as a result of the treaties that followed the end of WWII, became a territory governed by West Germany, an ally of the Western bloc, mostly represented by the USA and its interests. West Berlin was located right in the heart of East Germany, which was a satellite state of the USSR. This prompted the construction of the Berlin wall in order to prevent East Germans to flee into West Germany, particularly into West Berlin. This caused severe organizational and governmental problems for East Germany and the USSR all throughout the cold war era.