In the stories from The Adventures of Sherlock Holmes, while the main protagonist is Sherlock Holmes, the narrator is actually his friend and collaborator, John Watson. I would suggest that this decision actually makes a great deal of sense.
For one thing, there is Holmes' own personality to take into account, with his combination of intellectual brilliance and eccentric unconventionality. When writing from First Person Point of View, the author must essentially present said story from within that character's perspective, and Holmes, I imagine, would have been a very difficult character for an author to expect his readers to easily inhabit. In some ways, I suspect it was easier for Doyle to depict such a personality from an external point of view, through the lens of a more conventional and relatable personality such as Watson. Furthermore, viewed from such a lens, Holmes' own force of personality becomes all the more apparent, when presented within this contrast.
Additionally, it is worth factoring in that the Sherlock Holmes stories are mystery stories. As we will often learn after the fact, Holmes is continually testing various hypotheses or manipulating events through the use of deception. With that in mind, there is almost continually a deeper level of investigation transpiring beneath what Watson and the audience can easily observe. In most Sherlock Holmes stories, Holmes himself tends to be working several steps in advance of readers. To tell the story from his perspective would disrupt this sense of tension (although, certainly, there are cases where such an effect could introduce new sources of suspense and even ratchet up that tension in other ways).
With this in mind, I think locating the narrator in the personality of Watson makes a great deal of sense, and the stories are stronger for it. That being said, Holmes himself is such an interesting character, because his personality and mindset would have been so alien in the Victorian world, and being able to observe directly how he thinks and perceives that world would itself have made for a fascinating perspective. Furthermore, when faced with his more sinister antagonists (I'm thinking particularly of stories such as "The Adventure of the Speckled Band" and "The Final Problem"), telling the story from Holmes's perspective could have very well increased the story's suspense, given the the very real danger Holmes was facing, when confronting a character such as Moriarty or Dr. Roylott.