That's a change that has happened gradually over the last 4 or 5 decades I think. Guys like Camilo Pascual and Bert Blyleven used to make a living throwing curveballs, they must have been getting called strikes. Then the slider came along, late 60's maybe, and it was easier for umpires to call. Maybe over time they lost the ability to call the curveball, and pitchers stopped throwing it.