I think SFO's switches to western approach on cross runways during inclement weather with winds from the south or east. But the cross runways have lower capacity than the normal southern approach over water and also has lower limits so if wind speed picks up, they have to suspend operation. This is probably why a lot of flights end up diverting to LAX.
There may be more rain at LAX than SFO but LAX reverse flow doesn't reduce operating capacity and we don't have lower limits due to direction change. I can see long haul flights to SFO don't want to take the chance and be in hold for 4 hours.