Scout states that there is a "caste system" in Maycomb. How does she explain the system?

A caste system is one in which people are judged according to their status, whether it be related to social class, wealth, race, or religion. Even as a child, Scout sees clearly that such a system operates openly in Maycomb County.
The stability of the system depends very much upon people behaving in a way that is expected of them, based largely upon their family background. For instance, the Crawfords can never mind their own business; the Merriweathers have a tendency toward morbidity; the Delafields have a reputation for being less than honest.
Aunt Alexandra takes this notion to absurd lengths. She maintains that some families have certain "streaks" in them, character traits passed down from generation to generation. She is also something of a snob, believing that the longer a family has been established, the better they are. Jem contradicts Aunt Alexandra by citing the example of the long-established Ewells, a family known for being mean and lazy.
The apparent permanence of the Maycomb caste system helps to explain why Atticus Finch receives so much condemnation when he agrees to take on the case of Tom Robinson. By representing an African American in court, he is challenging the very foundations of Maycomb's rigid caste structure. Respectable white folk simply do not do that sort of thing.
At the same time, there is a sense in which the Finch family has never really been a part of the caste system in any meaningful sense; they all display such rare individuality, a quality seldom seen in other folks in Maycomb County.