Answer:
Because the Western powers were weakened and trying to rebuild after World War II.
Step-by-step explanation:
With the end of World War II, the Soviet Union now occupies the post of superpower beside the United States. Despite allies during the world conflict, the opposition between capitalism and socialism now divides the world into two major poles. Fearing a military confrontation, the two countries begin to invest in armaments and, still under Stalin's command, the USSR develops its nuclear program and obtains its first atomic bomb in 1949 in order to confront the Americans.
Faced with the military growth of the USSR in the postwar world, Western countries did little to stop the advancement and influence of the USSR in the world. This is because Western countries were weakened and distracted after the war, so these countries preferred to use their resources to rebuild themselves than to prevent Soviet advance.