All modern civilzation has been wiped out. All technology, books, computers, houses, microwaves, heating, clothing, everything...gone. The survivors, those who were left, had to leave the areas of radiation, taking nothing with them, and start over, completely from scratch. They couldn't whip out their heated blankets to stay warm or toss a pizza in the oven to eat. They didn't even have the basic supplies to create new technology, because it was all torched. So even though it is set in the future, the people have to start like the first of mankind had to start-with only their two hands. They live in the forests and hills, group together into tribes for protection, and create an entirely new society from nothing. Their housing is primitive, their stories of the past are mythical, and as generation after generation live in that lifestyle, the old ways are eventually forgotten. It isn't until John takes his journey that he realizes where they had all come from, and what a large and advanced civilization had been there. It will take thousands of years to advance to the levels that they were at before the bombs; in the meantime, they live where they are and will progress a step at a time.
good answer !