When I worked in ops, there were an almost infinite number of decision points.

It could have been that the crew didn't have the duty time left to fly to DEN. The fact that Frontier has a flight that they can book on definitely plays a part in deciding which flight to cancel, but I would assume with LAX-SFO being one of the busiest lanes in the country, it would have been the other way around.

Also keep in mind that almost everyone flying the LAX-SFO leg were either O&D or international connecting passengers. DEN is largely a domestic hub, which has far lower yields. Of course that's all speculation anyway.