In underhand we swing the arm around. The more it is away from the body, the more trauma to the shoulder and arm. It's close to the body providing more protection. One of the major concerns is also recovery time. In baseball pitchers usually only throw every 5 days.
Overuse then becomes a factor.
Arms are being used too frequently and are not given proper time to recover. Pitchers, especially young throwers, are throwing too much. They are throwing in games, in practice or at home or at the playground. In fastpitch the issue exists, but it is not so much of a problem because the arm can recover faster. The average fastpitch pitcher throws two to three times a week without any harm being done to the arm.