Great question. Funny answer. Maycomb is the county seat because an old tavern keeper named Sinkfield bribed and got drunk the two surveyors that were in charge of drawing up the county lines and choosing which town would become the county seat.
Maycomb should not have been the county seat because it is out of the way. It does not have good river access like Finch's Landing does. River access is important for trade and travel.
Sinkfield unfortunately had decided to build his tavern "awkwardly inland." Because of this, the surveyors were going to put the county seat in the town of Winston Swamp. Sinkfield got wind of this and selfishly "helped" the surveyors adjust their maps and county lines so that Maycomb was the best likely candidate for the county seat.
It was selfish because it placed his inn and tavern where the head of the local government would be. That meant more business for Sinkfield.
... lop off a little here, add a bit there, and adjust the center of the county to meet his requirements. He sent them packing the next day armed with their charts and five quarts of shinny...