Final answer:
Scout keeps her costume on after the pageant because she is excited.
Step-by-step explanation:
Scout keeps her costume on after the pageant because she is excited.
In the given passage, her father mentions that Mr. Gearson was in an excited state of mind and had set 'em up for the whole of Company A. This indicates that the pageant was a great success and Scout is thrilled about it. As a result, she wants to prolong the feeling and keep the excitement going, so she decides to keep her costume on.