Ah. I've never liked flagstone coping... it's always flaking and looks worn and bad quick. It would be an interesting conversation with a builder though!I assume it is because many use cheap flagstone around the pool. That flagstone degrades regardless of what chlorination method is used. But, if they install a SWCG, their customers will point to that as the cause because 'they saw it on the internet'. So, it is less hassle for the builders.
Builders have little clue on water chemistry. Or long term issues with the materials they use around a pool.