Why did they change the style for girls volleyball serves from fist to heel of hand?

Powerful Serve: Its an argument against underhand serves versus overhand serves. The overhand serve—generally faster and more powerful than its underhand counterpart—is far more aggressive and can be used as a valuable offensive weapon. By hitting the center of the ball with your overhand serve, creates more power and is harder to return.

More power: Hitting with the heel of the hand is a more solid and consistent joint area with more power. Hitting with the fist has more variables for error, such as hitting with the fingers that will also lessen your power on the serve.