A lotta mosques were saved for the same reason - you notice in Banda Aceh, only the big mosque made it. I think it's due to sturdy construction rather than the hand of God.
The mosques may have been saved but the difference is that the water went around the cathedral. As in, did not enter the Cathedral, where it did enter into the mosques. While the water went around the cathedral, it hit the other buildings on the same level.
I don't think that was due to sturdy construction.