S.O.S. a combination of letters it are very easy to transmit in Morse Code.
3 dots, then 3 dashes, and then 3 dots again.
On the other hand, it is distingishable, and will not be typed by mistake.

This was the reason they were chosen to symbol a distress call (a "call for help") when this was the only way to contact ship over seas.

The meaning like "Save Our Ship" or Soul came later, but are used just to remember and to bring some meaning the code.