Orwell has written a few different documents that address this.
First of all, Orwell set the book on purpose in Oceania. Oceania is made up of England, North America, and many places that have demoncratic regimes. He did this to prove that totalitarianism could happen anywhere. Our job as a reading audience is to take that warning to heart and heed it's caution. We need to constantly question our governments because even the greatest societies can choose to hurt their people.
When asked if his view in 1949 should have been viewed as prophesy for the world in 1984, Orwell wrote:
I do not believe that the kind of society I describe necessarily WILL, but I believe (allowing of course for the fact that the book is a satire) that something resembling it COULD arrive. I believe also that totalitarian ideas have taken root in the minds of intellectuals everywhere, and I have tried to draw these ideas out to their logical consequences.
Orwell wrote to clarify his purpose.
Furthermore, Orwell used 1984 to show that (especially in book 3) governments have a desire for more power and will often go after it at the expense of the people. When a leader gains power, there is a tremendous rush that comes with that. Orwell knew after watching several different vaccuums of power that someone will fill that void, often with great passion... for the good or bad, well, that is often up for a people to eventually find out.