I have a bit of a quirky and unsubstantiated theory (aren't those always fun). I believe that a huge factor in arm injuries is genetic makeup - simply the idea that some are born with ligaments that are better able to sustain high velocity pitching over longer periods of time without damage. In the past, pitchers threw a lot more at earlier ages. It was not unusual in high school to see a team throw a kid in back-to-back starts in back-to-back days well over a 100 pitches per outing. Back then, when you blew out an arm, you didn't go to the doctor and you certainly didn't even entertain the idea of surgery. You either pitched through it (ineffectually) or you stopped pitching. Injuries like this were undocumented (much like knee injuries in football in the old days). Today, we do so much better. We don't abuse pitchers the way we used to and when they are hurt, we get medical evaluations almost immediately. Heck, we even have a surgery for those guys with torn ucls. Back then, practically the only pitchers who made i tot the majors were those who were genetically gifted with the ability to have made it through the gauntlet of relentless overuse. Only the strongest survived. Today, we are able to keep kids healthy enough to reach their peak potential. We even often keep pitchers who, thirty to forty years ago, would have been out of the game by their teen years, healthy long enough to make the majors. This means a bigger pool of pitchers who can reach high velocities, but it also means more injuries as many of those kids are pitchers who were not genetically superior and, therefor, lucky to have kept healthy long enough to even get to the pros.