Why do we say "Elementary, my dear Watson"?

Meaning:

It's straightforward/simple

Background:

Almost everyone will recognize this expression as being from the mouth of legendary detective Sherlock Holmes. But it is not found in any of Arthur Conan Doyle's stories (published between 1886 and 1927). Instead, it seems that the first time Holmes expresses his signature phrase is in the 1929 film, The Return of Sherlock Holmes, with Clive Brook in the role of Holmes, released the year before Conan Doyle's death in 1930.

As The Phrase Finder points out, Conan Doyle gets very close to formulating the expression in various Sherlock Holmes stories, with Holmes describing events as "elementary" in "The Crooked Man" and addressing his sidekick as "my dear, Watson" on a number of occasions; but it took other writers to synthesize the expression we know today.

For example, Quote Investigator cites a 1908 article printed in a Rochester, New York newspaper describing a a system to send signals to the planet Mars using mirrors, and the unnamed writer opines that "It is such a simple little problem that any one should be able to take a pad and pencil and work it out in ten minutes. 'Elementary, my dear Watson,' as Sherlock Holmes was wont to say, 'Elementary.'"

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