Because people immigrated. Before the wall was constructed there was no (surprise surprise) wall to seperate the western side from the eastern side of Berlin which meant that the GDR citizens who lived in Soviet occupied Berlin, could go to and from West Berlin if they liked. But a lot of people from East Berlin used the open borders between the two parts to emigrate from East Germany to West Germany, where one of the reasons they did so was because of the higher living standarts in West Germany and also because West Germany was a democracy. This were really harmful for the eastern german society because among the people who immigrated to the west, a lot of highly educated people, who could be or already was very valuable for East Germany and the Soviet bloc countries. And because they couldn't really stop the emigration from East Germany, they descided to construct the wall.