As a likely tie-in to the upcoming movie, which stars Benedict Cumberbatch as the Marvel. Mordo: When the Sanctum is restored, they will attack again. Stephen Strange’s New York City lair is not quite so secret anymore. Then when the Hong Kong Sanctum falls the Dark Dimension is able to come through because the Sanctums can’t work together, because there is only one. With its Master gone, it needs another, Master Strange. You defended the New York Sanctum from attack. Only New York and Hong Kong remain now to shield us from the Dark Dimension. This still holds true as the New York and Hong Kong Sanctums still keep the protection active when the London One falls. Wong: A ritual to contact Dormammu and draw power from the Dark Dimension. A being of infinite power and endless hunger, on a quest to invade every universe and bring all worlds into his Dark Dimension. He is the cosmic conquerer, the destroyer of worlds. Wong: Dormammu dwells in the Dark Dimension. Strange: I just read it in the book of Cagliostro. Wong: Other-dimensional beings that threaten our universe. Mordo: The Sanctums protect the world, and we sorcerers protect the Sanctums.
Wong: Together, the Sanctums generate a protective shield around our world.
This is just a guess but considering the Ancient One specifically states ”together” I think it means that there has to be more than one Sanctum for the protection to work.