From chapter 5 of To Kill a Mockingbird by Harper Lee:
Miss Maudie had known Uncle Jack Finch, Atticus’s brother, since they were children. Nearly the same age, they had grown up together at Finch’s Landing. Miss Maudie was the daughter of a neighbouring landowner, Dr Frank Buford. Dr Buford’s profession was medicine and his obsession was anything that grew in the ground, so he stayed poor. Uncle Jack Finch confined his passion for digging to his window-boxes in Nashville and stayed rich.
Why did one stay poor and one rich? Please explain the lines in bold.