Well, the continental grip allows more wrist-forearm action. Also, as you pronate to strike the ball the pace is increased. Another reason is that all the men and women on the Pro circuit use continental grip. Since volley is associated with the serve and since continental grip is used for the volleys, conti is the most suitable grip for the serve also. The Eastern FH grip will put you in trouble in case you like to serve and volley. It's better in this case you use the same grip, continental grip.
In the past Pros, Boris Becker used to serve with close to Eastern FH grip.