Why was Louis Armstrong called Satchmo?

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Why: In The Stolen Child by Keith Donohue:

"Who's your favorite musician, Jimmy?"
"Louis Armstrong."
We laughed at the secret admission. In his waking life, he would have claimed some rock drummer like Charlie Watts of the Stones, but never Satchmo.

Answer: It's short for Satchelmouth!

The nickname Satchmo or Satch is short for Satchelmouth (describing his embouchure). In 1932, then Melody Maker magazine editor Percy Brooks greeted Armstrong in London with "Hello, Satchmo!", and it stuck.

Embouchre is the use of facial muscles and the shaping of the lips to the mouthpiece of a wind or brass instrument.

Source: Wikipedia

The More You Know: Early on, he was also known as Dippermouth. This is a reference to the propensity he had for refreshing himself with the "dipper" or ladle from a bucket of sugar water that was always present on stage with Joe Oliver's band in Chicago in the 1920s.