It's because, technically, Darkside is not a "stance". For SwO to be oGCD, you would be able to activate both ShO and SwO.
What they should do is simply buff PLD AA, remove SwO and bring back ShO to -20% damage to compensate (And maybe increase the cost of activating ShO).
SwO is the most bland stance since it's the only one to not change how you play your class at all. Even ShO synergize with Holy Spirit.