In short, George Orwell was a socialist but not a Marxist. One does not always follow the other. Here is an account from Isaac Deutscher on the specific gap between the Anglo-Saxon Left and Marxism:
Like most British socialists, Orwell had never been a Marxist. The dialectical-materialist philosophy had always been too abstruse for him. From instinct rather than consciousness he had been a staunch rationalist. The distinction between the Marxist and the rationalist is of some importance. Contrary to an opinion widespread in Anglo-Saxon countries, Marxism is not at all rationalist in its philosophy: it does not assume that human beings are, as a rule, guided by rational motives and that they can be argued into socialism by reason. Marx himself begins Das Kapital with the elaborate philosophical and historical inquiry into the ‘fetishistic’ modes of thought and behaviour rooted in ‘commodity production’ – that is, in man’s work for, and dependence on, a market. The class struggle, as Marx describes it, is anything but a rational process. This does not prevent the rationalists of socialism describing themselves sometimes as Marxists. But the authentic Marxist may claim to be mentally better prepared than the rationalist is for the manifestations of irrationality in human affairs, even for such manifestations as Stalin’s Great Purges. He may feel upset or mortified by them, but he need not feel shaken in his Weltanschauung, while the rationalist is lost and helpless when the irrationality of the human existence suddenly stares him in the face. If he clings to his rationalism, reality eludes him. If he pursues reality and tries to grasp it, he must part with his rationalism.
Source: Marxist Internet Archive