Why do most of the characters in To Kill A Mockingbird have nicknames?

In the South there is a custom of addressing people with
their first names. Usually, when a nickname is becoming of a person's characteristics
they go for a nickname that sort of identifies them. 

With
kids it is not an exception. We know from the beginning of the story that Scout's name
is Jean Louise and this is hardly a name that would fit her. She even admits to hating
it herself.

Jem's real  name is Jeremy and for a child that
is a bit long and serious to be addressed, so they shortened to Jem
.

However, it would be unfair to say that most of the
characters have nicknames because neither Tom, Mayella, Mr. Ewell, Calpurnia, nor the
aunt are addressed by nicknames, but respectfully by their respective
names.

Hence we could safely assume that the practice is
one specifically given to children to shorten their names and allow them a level of
individuality.