[steve1977]

Interestingly there were competing distress signals at the time, but I guess SOS won because of its usefulness outside of morse code. As you mentioned, it's easy to remember, the letters are distinguishable and it also works upside down when written.

[howard941]

CQD lost out

[nhecker]

As did OOO.

[austinallegro]

OOO = Out Of Office

[linzhangrun]

Titanic sent CQD first, then tried the new SOS