1. It's human nature to want to solve puzzles and mysteries. We like the challenge and suspense, but we also believe in our ability to solve them--faith in reason, facts, science, technology.
2. The imagery and concrete, specific details of the stories captivate us. The plots demand our attention, and we have to follow the clues. We get to be the detective.
3. Sherlock Holmes is an accessible character (in what sense?) that uses his brains. He highlights intellect and knowledge.
4. Sherlock Holmes understands human nature and the patterns of life, the world around him. He sees and observes what we don't. He reasons (deduces).
5. Holmes is like a superhero (see #3), and Watson is his sidekick. We need heros. Watson is also the narrator/writer.