Why does Doyle choose to have the story narrated by Dr. Watson? (chapter 15)

While Holmes is a great interpreter of facts and deduction, Watson is a very reliable narrator. Holmes can be detached from emotion and social nuances. Watson understands these things. Watson has common sense but also a domesticity about him that people feel they can trust.. Holmes serves as a surrogate for the reader. He can sometimes be described as rather a flat character so the reader can easily fit into his shoes.