Because objectivity vs. subjectivity is not the same as truth vs. untruth if we're going to split epistemological hairs.
Winston is searching for truth and that quest for truth in the way men are wont to do. Not only by trying to find documented differences between the historical record/geopolitical reality and the state narrative, but also grasping and experiencing life and love. Rich experience the State deprived him of.
Ingsoc doesn't propagate subjectivity. It propagates lies.
I think subjectivity IS lies. Truth is objective only. I don't believe in subjective truth. I see this as the whole theme of the book- truth is objective only.
If we believe in subjectivity, we are saying that truth is not absolute and is subject to perception. I think Orwell is saying just that: truth is objective- it stands outside of people and perception. It is external- not internal. As Christians, we believe in objective, absolute truth. There is no such thing as subjectivity in Christianity. What is truth? It is Christ- external, not subject to our perception.
I am jumping ahead but I want to quote these passages to demonstrate my point that I think Orwell is arguing against the idea of subjectivity. That's what doublethink is, making truth subjective.
"Doublethink means the power of holding two contradictory beliefs in one's mind simultaneously, and accepting both of them. The Party intellectual knows in which direction his memories must be altered; he therefore knows his that he is playing tricks with reality; but by the exercise of doublethink he also satisfies himself that reality is not violated. The process has to be conscious, or it would not be carried out with sufficient precision, but it also has to be unconscious, or it would bring with it a feeling of falsity and hence of guilt. Doublethink lies at the very heart of Ingsoc, since the essential act of the Party is to use conscious deception while retaining the firmness of purpose that goes with complete honesty. To tell deliberate lies while genuinely believing in them, to forget any fact that has become inconvenient, and then, when it becomes necessary again, to draw it back from oblivion for just so long as it is needed, to deny the existence of objective reality and all the while to take account of the reality which one denies- all this is indispensably necessary."
Subjugating objective truth and making it subjective.
"Only the disciplined mind can see reality, Winston. You believe that reality is something objective, external, existing in its own right. You also believe that the nature of reality is self- evident. When you delude yourself into thinking that you see something, you assume that everyone else sees the same thing as you. But I tell you, Winston, that reality is not external. Reality exists in the human mind, and nowhere else. Not in the individual mind that can make mistakes, and in any case soon perishes; only in the mind of the Party, which is collective and immortal."
Reality, truth, is objective and outside perception. Truth is not subjective nor can be. If it is subjective it is no longer external.
Maybe I am in the minority, here, in believing truth is always objective- but I think that is what Orwell is saying. It is objective and not open to manipulation, whether anyone believes it or not.