The thing that jumped out at me, and isn't explained in this article or the linked ones, is that all of the distress-related prowords are French and none of the others are. Specifically, they're French-derived but spelled as if in English ("MAYDAY", "SEELONCE").
See also PAN-PAN, from the French 'panne' meaning breakdown.
The story I was told behind the origin of these was that they evolved from British <-> French air traffic and was a reasonably logical thing that everybody agreed on and could pronounce intelligibly for the other party.
Edit to add: English is the official international language for naval/aviation communication. Having really important words in another language makes them stand out and hard to say by mistake.
- French was the language of the ITU and its predecessors.
- On HF radio you need words that can be distinguished very clearly and not mistaken for somethkng else.
I think these particular words are older than many of the other ones. Also French-derived is CQ, which comes from the French sécurité. These phrases date back to when French was used much more as an international language.
Why?