B737-112: I know it's incorrect; I thought you meant you didn't know from where the acronym came.
In CA, many of us (even non-aviation buffs) refer to SFO as "SFO". On the other hand, I know of no one who calls San Diego "SAN".
I don't think anyone calls John F. Kennedy by its full name. Instead, I hear "JFK" and “Kennedy”.
I think it all comes down to how simplistic the name is to say. If it rolls off the tongue quickly and effortlessly, perhaps people will call it by its name. And if not, I guess folks just get creative with what they use to describe it.
I love America. I guess that makes me Bush's poodle, but I'd rather be a dog in New York City than a prince in Riyadh.