Michael, what a great question.
I have struggled to decide how many people the Earth can support sustainably. We were told to be optimistic and I think the figure chosen probably is. The story is set some time in the future, falling birth rates, disease, famine, and war have reduced the population to near the 5 billion housed in the sanctuaries. (It says- disease, famine, and war were left behind)
There are still several reasons for people being outside of the sanctuaries.
Some people thrive on challenge, pitting themselves against adversity or foes and conquering. Either conquering the environment (e.g. mountain climbing, jungle survival ) or own fears or limitations or their own personal best; winning in a contest or battle. For the joy of overcoming, the adrenalin, the endorphins, the camaraderie and celebration. That's what makes life meaningful and worthwhile for them. Being safe and having an easy life could seem suffocating and prison like. If these people are enclosed within the sanctuaries they could upset the harmony because they do not have a dedication that makes their heart race and that bonds them closely to like minded souls, and that they feel allows them to express their true warrior nature.Its better that they serve the sanctuary rather than act against it. So there is some reciprocal advantage to the arrangement. Food and water supplies, holidays in the sanctuary, respect, admiration.
Social cohesion is very important but some may resent the social affirmations which are a kind of drip fed, daily indoctrination, or they may disagree with the imposition of compulsory sleep or with the planned parenting measures.Strong minded individuals particularly those wanting personal power may exclude themselves from the sanctuaries. Anyone can choose to leave, it isn't a prison. Some may get personal fulfillment and purpose from banding together with like minded individuals, struggling to survive and working against the sanctuaries.In the name of freedom but wanting control of the resources and to be in charge. These people were referred to as terrorists
Another group are criminals that have incurable personality disorders.Rather than having people permanently incarcerated for the safety of the sanctuary they will be exiled. Unlike the terrorists who just want to break the system the criminals may just want to get back into the sanctuary where life is a lot nicer or to get resources as they are scarce outside.Finally there may be feral bands that are the descendants of those that did not want to enter the sanctuary because its culture conflicts with their strongly held beliefs. They hang on to that identity because it is all that they have.
Why cannot all of humanity find sanctuary in the future? In a nutshell, certain aspects of human nature and personality. They do not want sanctuary or can not be given sanctuary for the safety of the rest.