Why did the soviets finally cut off berlin from western europe?

The Soviets sought to create a unified but demilitarized Germany under their tutelage, or as Soviet Foreign Minister Vyacheslav Molotov told U.S. Secretary of State James F. Byrnes in 1946, a united Germany that could be neutralized after Russia received industrial reparations from Germany. This strategy was a response to a 150-year history of repeated Western assaults on Russia, including World War I and Napoleon's 1812 invasion. Stalin considered it essential to destroy Germany's capacity for another war, which conflicted with the U.S. desire to rebuild Germany as the economic center of a stable Europe. (Stalin assumed that Japan and Germany could menace the Soviet Union once again by the 1960s.)
On June 24, 1948, the Soviet Union blocked access to the arteries of the three Western-held sectors of Berlin, which was deep within the Soviet zone of Germany, by cutting off all rail and road routes going through Soviet-controlled territory in Germany. The Western powers had never negotiated a pact with the Soviets guaranteeing these rights. Amid the fallout of the London Conference, the Soviets now rejected arguments that occupation rights in Berlin and the use of the routes during the previous three years had given the West legal claim to unimpeded use of the highways and railroads.