Discussed to death, but basically, it is in fact a conspiracy.

Lets use United as an example. United gets (and I am totaly making this up) 20 flights per hour into SFO. When the Fog roles in, they can only bring in 10 planes. So they cancel all the commuter flights from LAX and DEN, and leave the BIG planes to land at SFO. If its really bad, sometimes you get to hop on a 757 / 767 thats coming in from "somewhere else" that lays over at LAX and then continues on to SFO. Or, they will cancel 4x LAX-SFO flights on RJ's and send everyone up to SFO on a 747. (10 hours late of course).

Southwest does not have this flexibility. They only have 737s and they only fly short haul. Plus they cant easily change their route or cancel the Regional jet flights. So, they do ground delays. You wait until they can land, usually an hour or so after the scheduled landing.

So, you are correct, its a conspiracy. United cancels their short haul flights when the fog rolls in, and WN sits on the ground waiting to take off.

This is why "I Fly OAK"... :-)