why did mr Morrison decided to live inthe deserted shack

Mr. Morrison is described as a quiet men who loved his privacy. The shack would give him both.

"But Mr. Morrison was a quiet man, almost shy, and although Mama had offered him lodging in our house, he preferred the old shack. Mama sensed that Mr. Morrison was a private person and she did not object to the move, but she did send the boys and me to the house to help clean it."