I don't know if subconsciously I had heard these stories before, but the mystery in a lot of these mysteries didn't seem so mysterious.

If you remove "someone impersonating someone else" and "we know who did it and why, the only surprise is the specific details of the murder that you can only kind of guess at with the detail provided" then you rule out the vast majority of these stories.

But to be fair 'Scandal in Bohemia' was great, and I may be coming in with grandfathered expectations.