Answer:
He wanted to keep skilled East German workers from moving to West Berlin.
Explanation:
To impede the constant crossing of large numbers of East Berliners and East Germans to West Berlin, controlled by the Western powers, was the main motive behind the construction of the Berlin Wall in 1961. It meant to stop the loss of skilled workers, too. This measure was decided by the German communist authorities and it must have had Soviet approval. Nikita Khrushchev was the Soviet top leader at the time.
