I agree a major reason is overuse at a young age. It used to be we played baseball in the spring, football in the fall and basketball in the winter. Now a kid will play baseball in the spring, then a summer league followed by a fall league until late October. Then they have workouts from November until school tryouts again in February.
Also, they used to say not to throw anything but a fastball until 15 or 16 years old. I know Mike Mussina said he never through a fastball until then. Now when a 13- or 14-year-old tries out for pitching they want to see his curve.