Why did Arthur Conan Doyle make Dr. Watson narrate this novel instead of making Sherlock Holmes do it himself?

Sherlock Holmes, while much more attracted to the spotlight than he might let on, is not one for narrative flourishes. Violent emotions and sentiment are not his strong points. He is all about the cold facts, picking up details and following them to their logical conclusion. While this make him a great detective, it makes him a poor story-teller. One just has to take a look through "The Adventure of the Lion's Mane," narrated by Holmes rather than Watson, to see how unconcerned Holmes is with injecting any enhancing flourishes that would turn it from a case to a story. For example, there is a huge chunk of dialogue because Holmes is simply recording the event verbatim, offering no inner reflections or observations, no thought process behind why he chooses his questions. To be fair, giving voice to too many of his thoughts would likely result in the mystery being solved for the reader, but that's just the point- Watson's narration allows the mystery to long remain a mystery while still having the opportunity to add his own nervous, awed, or other emotional impressions of the events to make it read like more than just a straight case file. Watson is in a sense the heart to Holmes' brain, giving Watson all the "silly" human feelings that help to color the stories. Even Holmes, who often chides Watson for the way he records their cases, cannot help but acknowledge the unfortunate lack of Watson's narrative flair.

"Ah! had he but been with me, how much he might have made of so wonderful a happening and of my eventual triumph against every difficulty! As it is, however, I must needs tell my tale in my own plain way..."