One can only assume it is out of politeness for the man's personality. Scout wonders the same thing herself and says, "I wondered why Atticus had not brought a chair for the man in the corner, but Atticus knew the ways of country people far better than I." She goes on to insightfully observe that "this one was probably more comfortable where he was." As we find out in the next chapter, this houseguest is none other than Boo Radley, and yes, he was probably much more comfortabel hiding in the corner with no one paying him any attention, then being forced to sit down with the others and converse. He hasn't socialized with other people almost his entire life; he has remained closeted in house, and isolated. So, Atticus not drawing attention to him was a polite way of respecting where Boo had come from. They are all civil, but don't want to make him uncomfortable. So, they let him stand where he was, which was a very kind thing to do.
Atticus didn't bring a chair for the man in the corner, because that man was Arthur "Boo" Radley, one known for staying to himself. Despite the children's hypothesis that he was a raving lunatic surviving on the blood of raw squirrels, in reality he was a shy intraverted fellow who kept to himself due to having been shunned by the town years ago.
Boo would've felt uncomfortable talking much with anyone, even Atticus and the Sheriff. In fact, Boo never says a word; he just sort've motions and uses facial expressions. From the years of confinement, it's possible he hasn't said a word in a very long time.
This is just another example of the respect Atticus shows his neighbors and fellow townspeople, due to his keen sense of perception of others.
He was a country folk and Atticus had worked with them a lot and knew he was comfy just leaning against the wall. that stranger is no stranger. He is Boo Radley and he had saved Jem and Scout from Bob Ewell.
This was Boo Radley. Boo Radley was not some type of monster, but a person who had become very introverted because of his closure from society. Since he had not been social because of his father and brother, sitting on the side was comfortable for him. This in fact was very kind because then Boo would not feel uncomfortable.