I've been shooting targets since long before sporting clays gained popular in the US. It's my experience that the shooters who flocked to shoot sporting clays were those who didn't consider themselves good enough to shoot NSSA and/or ATA events. Or, they had tried trap and/or skeet and didn't do very well. Ask my friend, 1-oz, about his highest class shooting ATA or NSSA.
Trap and skeet are like bowling in that there is a perfect score and it's attainable if you work hard enough. Sporting clays is like miniature golf, there's enough craziness to make perfection virtually unattainable.