It’s one of those weird things we do in America. San Francisco is SF, Los Angeles is LA, San Diego is SD, San Antonio is SA, and Dallas is DFW. You might hear those said in conversation. Austin is ATX, Houston is HOU, and Portland is PDX. Those are more likely to be referenced in written text rather than said out loud.

Not all of those are airport codes. I suppose it’s due to the fact that Portland is just one word, and it doesn’t contract as nicely as the other cities.