There are two reasons why the main character, John, speaks English in such a strange way.
First of all, I think that the author is trying to sort of trick us at first. I think he wants to make it seem like John and his people are some sort of primitive tribe from the past. They seem a lot like Indians from the time before European contact and I think that his strange English is supposed to heighten that feeling in us.
Second, he speaks strange English because he is actually living in a time far, far in the future. Languages change over time just naturally and English (in this story) would have changed a lot. After all, it's been so long that people don't even realize that New York City was a city of people made by people. So you know that a tremendous amount of time has passed between the author's time and the time when this story is set. Therefore, it is natural that the English spoken by the characters would have changed.