Mainly due to ideology, the Soviet Union was dedicated to expanding communism and during the war the Soviets were needed to resist German ambitions but they did so at the expense of a lot of territory that was previously not under communist influence, fearing revolution the US and British Empire (or rather what was left of it) which were the primary capitalist societies feared the destruction on the continent might cause communist sympathy and so the Marshall plan rebuilt Europe quickly while containing the Soviets in the East. Interesting Churchill actually had plans to launch a preemptive invasion and there were some calls for the Americans to use nuclear weapons before the Soviets caught up in terms of technology.