Why couldn’t Leslie go to Terabithia that weekend?

In Bridge to Terabithia, Leslie cannot go to Terabithia on the weekends because Mr. Burke is in the process of repairing the old Perkins place and wants Leslie to be around. Mrs. Burke is busy writing a book, so she is unable to help. This leaves Leslie responsible for "hunting and fetching" for her absentminded father. Leslie has a talent for finding the items that he misplaces, and her father enjoys having her for company. Additionally, Leslie loves fixing up the house, appreciates being needed by her father, and enjoys the fact that she is gaining a better understanding of her father. 
This is troublesome for Jess, who finds it difficult to create the same sense of magic in Terabithia when he goes there alone; he believes that the magic is "reluctant" to come to him when Leslie is not there.