Originally, when the US didn't have as many airports, the airports were identified with two character codes, so Los Angeles was "LA", Portland was "PD", etc. When the US had to go to 3-character airport identifiers, the "X" was added as a place marker that identifies the "Legacy" airports, so LA became LAX, and Portland became PDX.

LAS could not be the airport code for LA, because it is LOS Angeles. LOS is already in use by the airport in Lagos, Nigeria.