TL;DR: because mid-20th-century designers believed soft green reduced eye strain and improved focus. Basically the same nonsensical belief as in regard the dark mode nowadays. I don't even believe it's true. Green is just an army colour, that's pretty much it. Army uses army colours. Mystery solved.
Maybe it even works better with the color of a clear blue sky above it.
Anyway, it's intuitive and not rocket science.