Sorry, another Sherlock Holmes related post today. But this post is on the original books, rather than TV or movie adaptations.
I suppose it can be applied to the adaptations as well though. This is because we know little about John in the original books. Instead, Sherlock is the character that’s more in the narrative spotlight.
Also, it can be said that the only side of Watson that we know about is that of his love life (in particular, his relationship with Mary Morstan). Aside from that, his role in the books can be interpreted as being more of catalysts for the plot’s motion, rather than a character to be explored.
But, when it comes to adaptations, I suppose this creates the opportunity to create sub-plots throughout a series where Watson’s character is developed. If anything, it allows scriptwriters to develop the character of John so that he is not portrayed as an everyman figure.
Obviously though, this lack of involvement by Watson means the attention is shifted to Sherlock (further supported by Watson’s narrative tales on the character). This is probably why Sherlock Holmes has gained more fictional fame alone, rather than with his sidekick, John Watson.