In my opinion, the book has been successful because many people feel that they are being oppressed by the dominant, hegemonic culture.
Freire's book argues that people are oppressed, in part, because they have bought into this hegemonic thinking that the dominant classes and countries support. These ideas justify the fact that the poor are poor and the wealthy are wealthy. When the poor buy into them, they participate in their own oppression.
What this book offers "oppressed" people is an argument about why they are oppressed. It tells them that their problems are the fault of others and that they need to rebel against these hegemonic forces.
This would be a very attractive argument to people who feel that they are oppressed. I think that is why the book has sold so many copies.