The explanation for why Sam and Bill choose Summit as the location of their kidnapping scheme occurs in the first three paragraphs of the story. Since the story is told in the first-person point of view by Sam, who is an unreliable narrator, readers must take what he says with a grain of salt and read between the lines to get the real meaning. Sam gives a few reasons for choosing Summit and hints at others.
The reasons Sam gives for choosing Summit are that "philoprogenitiveness ... is strong in semi-rural communities" and that the law enforcement and journalism in the town are inept. The concept that philoprogenitiveness, or loving one's offspring, varies by type of community is ludicrous on its face. People in the city and in the country love their children as much as those in "semi-rural communities," so this explanation is more of a fabricated rationale than a well-considered reason. The real reason for choosing Summit seems to be that it was convenient to where they were at the time when they hatched the plan. They needed money to carry out their fraudulent scheme in Illinois, but they only had $600 between them, so they wouldn't want to waste their funds on travel.
Sam suggests that "other reasons" besides philoprogenitiveness were behind the decision. These seem to be the lack of competent policing and the shortage of hard-hitting journalists in Summit. Sam and Bill thought they could easily escape the "constables" and "lackadaisical bloodhounds" that might come after them. And rather than dispatching "reporters ... in plain clothes" like the city newspapers sent out to "stir up talk about" kidnappings, Summit was only likely to publish "a diatribe or two" in the weekly farm paper.
In addition to these stated reasons, there is another hint from Sam as to why they chose Summit. They believed the inhabitants of the town were "undeleterious," that is, harmless, and that they were self-satisfied, meaning that they might be so content with themselves that they could be easily deceived. How did Sam and Bill reach this conclusion about the residents of Summit? Perhaps simply because of the fact that they had named their town Summit even though it was "as flat as a flannel-cake." It's possible that Sam and Bill figured that people who would name a flat town "Summit" could be easily manipulated. Unfortunately for Sam and Bill, that decision, made "during a moment of temporary mental apparition," turned out to be a poor choice. They jumped to a conclusion about the people of the town but met their match in Ebenezer Dorset.