SFO is notorious for weather delays for all airlines. That being said, other airlines also perhaps have more flexibility. I believe United and Delta fly mainline aircraft between SFO-LAX. When delays occur, airlines often decide to delay/cancel flights on smaller regional planes in favor of trying to keep mainline aircraft closer to scheduled times (theoretically inconveniencing fewer people). Southwest, operating all of the same type of plane, likely has to use other factors in determining which flights to cancel, delay, or prioritize.

My hypothesis for one of these factors is that they shuttle some of the planes to do a few LAX-SFO trips back and forth during the day, and are more likely to cancel one of those than a flight that's scheduled to go to other airports. (e.g. If a plane was scheduled to go SFO-PHX-DAL-DCA, and another plane scheduled to go SFO-LAX-SFO-LAX, cancelling the first SFO-LAX flight would have less of a downstream effect than cancelling the SFO-PHX flight).