The jury sat to the left, under long windows. Sunburned, lanky, they seemed to be all farmers, but this was natural: town folk rarely sat on juries, they were either struck or excused. One or two of the jury looked vaguely like dressed-up Cunninghams. At this stage they sat straight and alert.
In Chapter 16 it is Scout speaking as the narrator who states that jurors are generally from the country and not from the town as town people are "either excused or struck" from the jury list. It may be assumed, though, that Scout learned this from Atticus, as she learned so much else. No further information is given as to the reason that this exclusion for town people exists. Therefore, readers are left to infer the reason on their own.
Since the chapter has a great deal to do with the theme of inclusion and exclusion from society, and since it follows immediately after the meeting between the children and the crowd at the jailhouse, we can infer that there is a symbolic representation in this short remark by Scout. At the jailhouse, Lee demonstrates in Chapter 15 the violence that attends people who are excluded and who break the social code, like Tom is supposed to have done. Chapter 16 demonstrates the order that is meant to reign over those who are excluded and included within society and who keep society's codes. Just like in the courthouse, violence is kept down when social codes are kept unbroken.
This double representation can be applied to the question of why town people are not selected to pass judgement as jurors upon their fellow guilty citizens in court hearings. The fear and threat of violence to the town, the symbol of order and stability, is too great to risk if a town citizen should sit on a jury. It is because of this fear and potential threat that a town juror might be too easily coerced and bribed into giving a verdict that would uphold peace at the expense of denying justice. In other words, we might infer based upon this symbolic representation that the risk of retaliation and violence to the town was too great for a town juror to be asked or trusted to give a truthful verdict.
Atticus says that the jurors are always from the country because there would be a conflict of interest among the people of the town. For instance, if the owner of the town grocery store was on the jury and sided with the defendant or the prosecution, then the town grocery store may lose business and customers if others did not like the way the grocer voted during a trial. This could be said with any business person in town.