In To Kill A Mockingbird, Scout tells us that when Calpurnia stays overnight at the Finches', she sleeps on a cot in the kitchen. Why do you think...

Calpurnia’s cot is an example of how segregation extends into every corner of Maycomb, including the Finch house.

The Finches obviously have a spare bedroom.  Aunt Alexandra has to sleep somewhere when she comes to stay with them.  When Calpurnia stays at the house, she does not sleep in a spare bedroom.

When Calpurnia stayed overnight with us she slept on a folding cot in the kitchen; that morning it was covered with our Sunday...

Calpurnia’s cot is an example of how segregation extends into every corner of Maycomb, including the Finch house.

The Finches obviously have a spare bedroom.  Aunt Alexandra has to sleep somewhere when she comes to stay with them.  When Calpurnia stays at the house, she does not sleep in a spare bedroom.

When Calpurnia stayed overnight with us she slept on a folding cot in the kitchen; that morning it was covered with our Sunday habiliments. (Ch. 12)

Why does Calpurnia sleep in the kitchen?  She is black, and the Finch family is white.  As egalitarian as Atticus Finch is, he is probably not going to let a black woman sleep in a regular bedroom in his house.  It would attract attention.  After all, his sister Alexandra would probably have a fit about it.

Although Cal is treated well and almost as an equal by Atticus, she is not completely equal.  There is a great divide between Calpurnia’s personal life and her professional life as the Finch family housekeeper.  At the black church, Lula confronts Calpurnia for bringing her charges—her white charges.

“I wants to know why you bringin‘ white chillun to nigger church.”

“They’s my comp’ny,” said Calpurnia. Again I thought her voice strange: she was talking like the rest of them.

“Yeah, an‘ I reckon you’s comp’ny at the Finch house durin’ the week.” (Ch. 12)

Scout is surprised that Calpurnia's diction changes when she is at church.  She speaks like the rest of the white people at the Finches' house most of the time, but when she is among her fellow black community speaking like that would seem like putting on airs. 

Another example of this is when Scout asks to visit Calpurnia’s house.  Calpurnia is dismissive to her.  There is a segregation between blacks and whites in Maycomb, and it is very hard to break through.