Arthur was locked in the Radley's house because his father thought it was a disgrace to send him away.
Step-by-step explanation:
Arthur Radley who was also known as Boo Radley was locked up because he was involved in a crime. The crime that he had committed was that he along with his friends had stolen a car and then he had also locked up the officer in the courthouse.
Boo was locked away due to his association with "an inappropriate group"; his dad felt that Boo being placed in a reformatory would be a humiliation to the family. In any case, the fundamental explanation is a result of Mr. Radley's strict enthusiasm.