Jem is right on target when he describes what he believes to be the social order in Maycomb.
"There's four kinds of folks in the world. There's the ordinary kind like us and the neighbors, there's the kind like the Cunninghams out in the woods, the kind like the Ewells down at the dump, and the Negroes." (Chapter 23)
Although the Ewells are the "disgrace of Maycomb for three generations" and the epitome of Southern white trash, they are still white, putting them above Tom Robinson--and all other African Americans--on the Maycomb social totem pole. Most of the Ewells' testimony is untruthful, and Tom's seems to be both honest and heartfelt, but in 1930s Alabama, a white man's word is always believed over the word of a black man, and
"The jury couldn't possibly be expected to take Tom Robinson's word against the Ewells'..." (Atticus, Chapter 9)
Atticus explains that many townspeople and business owners refuse to serve on juries, leaving rural residents like the Cunninghams to populate them instead. Additionally, women are not yet legally allowed to serve on Alabama juries, so people like Miss Maudie--who would have no doubt voted with her conscience and not by skin color--are absent.