Why did Jean-Paul Sartre believe that Marxism needed to account for individual freedom and specific social conditions?

Jean-Paul Sartre believed that Marxism needed to account for individual freedom and specific social conditions because he observed that the version of Marxism practiced in the Soviet Union had become ossified. He argued that instead of adapting to particular situations, this rigid form of Marxism forced unique, concrete circumstances to fit a predetermined universal model.1

Reflecting on his experiences during and after World War II, Sartre concluded that his earlier, prewar philosophy of radical freedom was naive and failed to address the realities of history and social justice. In his major work, Critique of Dialectical Reason, he sought to reconcile his existentialist focus on the individual with a Marxist philosophy of history. He maintained that while Marxism remained the essential philosophy for the modern era, it must learn to respect individual freedom and recognize the distinct existential circumstances that vary from one collectivity to another.231