Hi, I've been drinking, so I apologize beforehand for spelling or grammatical errors.

A little about the era before the wall was errected: This was a period in East German history ruled by Stalinist individuals, people who were basically in the pocket of Joseph Stalin and willingly subscribed to his brutal variety of socialism. This meant several things. One was that the arts and culture was subdued if it was not congruent with the socialist ideal; this meant that artists fled in great numbers to the West. Science was also subject to great scrutiny. Job protection for academics was not based off tenure, but rather if they defended the administration. "Subversive" science was a common accusation that was leveled amongst legitimate academics and often cost them their jobs if they didn't toe the line; so academics were another large group that fled. Lastly, people who were white-collar workers, meaning non-industrial. Doctors, lawyers, researchers, entrepreneurs. All of these individuals feared Stalinist reform to their particular professions. Doctors would be forced into state medical service rather than private practice. Lawyers would be practicing in a form of law and order that they were not trained in. Researchers, like academics, would be expected to perform research according to (and supporting) the new Soviet ideal. Entrepreneurs were pretty much screwed. The state seized control of most industrial activity and there was not a lot of benefit for these individuals.

This was called the "brain drain". As you can assume, the skilled and intelligent were the main group to flee the GDR. This was a huge problem for the Socialist Unity Party (SED) regime, as they were left with academics/doctors/etc who were there either out of necessity, fear, or loyalty. Often people could not flee due to family matters, would not because of fear (understandable given the fall of the Nazi regime making them scared of government), or loyalty to the new socialist government (rare).

What we found was a group of people either trapped or willing to live in the new polity. Many people bought in to the promises of the regime; the poor, the unemployed. The pacifists were a large portion who stayed. Germans were exhausted from war at that point, can't really blame them. Two world war losses, occupation by foreign powers, it was a hard life. There is qualitative evidence showing that the German people were psychologically beaten and downtrodden. Most were scared. The Red Army remained in the GDR for a long period of time, the Ulbricht regime created the Stasi early and suppressed opposition after the 1953 revolt with brutal efficiency. Even the change of Soviet government from Stalin to Khrushchev didn't even help, Ulbricht was still a Stalinist dick to his people even though Khrushchev pushed reform. Authoritarianism worked in the GDR and the SED didn't want to change.

tl;dr: Fear, war exhaustion, ignorance, and concern for one's family caused Germans to stay in the GDR.

Read: McAdams "East Germany and Detente", Applebaum "Iron Curtain", Sarotte "Dealing with the Devil", Fulbrook "Anatomy of a Dictatorship"