In addition to be so well respected, Atticus has deep family roots in Maycomb County. In Scout's recitation of family history in the novel's first chapter, she tells of their ancestor, Simon Finch, who settled Finch's Landing well before the Civil War, so far before the Civil War in the 1860s that Simon didn't live to see it. The Finch family has been in Maycomb County before there was a Maycomb County. Scout says that Atticus was "Maycomb County born and bred." He knew "his people" very well, she says, and "they knew him." Scout also points out that "Atticus was related by blood or marriage to nearly every family in the town."
As a man of integrity without prejudice, Atticus Finch is respected. A mature, rational man, he is what the highly emotional people of Maycomb need. His demeanor in the courtroom exemplies his fairness, tact, and understanding; he is one of the few people in the town who can handle both black and white citizens fairly. In fact, Atticus is probably the only man in town who can defend Tom Robinson on the basis of justice alone, not allowing anything about Tom to prejudice him.
In To Kill a Mockingbird, Maycomb County re-elects Atticus Finch every year because he is a well-known, permanent fixture of the town; his historical roots extend far back in time in the area, as one of his ancestors, Simon Finch, settled the area before it even became known as Maycomb.
Not only is Atticus part of a well-known family, but he is also a man held in high esteem in the town. He is largely recognized for his intelligence, thoughtfulness, integrity, and respectability. Even if the town may not agree with all of his political or personal decisions as an attorney--especially not his choice to defend Tom Robinson--he is still so well established within the community that it does not really make a difference. Maycomb thus seems to operate on a simple idiomatic premise when it comes to this appointment: "Better the devil you know than the one you don't."
Atticus is a great lawyer therefore, gets elected every year.
Because the town knows that Atticus is smart and he is a great lawyer so they re elect them.