Thanks to Steven Greydanus for catching this revealing admission from Alfonso Cuarón regarding Children of Men:
Filmmaker: Was there a script already written when you came on?
Cuarón: There was a script and I read the beginning of it and didn’t like it. I wasn’t interested in making a science fiction film and secondly I wasn’t interested in the environment that the book takes place, all this upper class drama. For me it was more important to explore the thematics that are shaping our contemporary world. The P.D. James book is almost like a look at Christianity, and that wasn’t my interest. I didn’t want to shy away from the spiritual archetypes but I wasn’t interested in dealing with dogma.