As the initial chapter of this monumental text makes clear, Winston Smith is writing in a world where every single move and action of citizens is scrutinised and examined for any deviant behaviour. The results of indulging in such behaviour will be instantaneous and ruthless, as the references to numerous citizens being vapourised make clear. In addition, the possession of a diary is something that Winston himself recognises could land him in considerable trouble, as it is an object that comes from history and is not owned by the majority of people.
Living in such an environment when the Thought Police are believed to be able to enter your thoughts and detect rebellious people, writing your thoughts and subversive ideas down in a diary that may be found and used against you is foolhardy in the extreme. To consider how a rebellion might be mounted in your mind is one thing, but to openly write about it and entertain such notions in written form, which could be discovered, would be a suicidal act.