Why did Orwell choose Winston as his protagonist in 1984?

In 1984, Winston Smith is employed within the Party's propaganda department, the Ministry of Truth. His job is to ensure that all existing documentation matches the aims and beliefs of the Party, to create the perception that the Party is consistent in its views; this involves changing documents, erasing the names and photographs of individuals who the Party deems "unpersons," and generally rewriting history. Having the main character serve this role is important to the story, because Winston has more insight into the truth about the world he lives in than others employed in different sectors of the Party. It is important throughout the story that Winston is a person who is aware of the Party's propagandizing, lying, and revisionism; it is the knowledge he gains from doing his job that allows him the awareness he needs to begin secretly hating and rebelling against the party. If, for example, Julia were the narrator of the story, there would be no insight into the process of revisionism; Julia hates the oppressive government too, but her job doesn't put her in a position of constant awareness of the ways in which she is being lied to.