By diving on the goalkeepers' side this allows the goalkeeper to protect the body and the goal. There are less sensitive impact zones such as organs or joints on the side of the body.  The body and neck absorb the impact with the ground on the side resulting in a softer landing.  Diving to get more of the goalkeepers' body behind the ball also helps secure the ball better by allowing two hands to get to the ball.  Two hands are better than one and if the ball is in the goalkeepers' hands the ball could be used to potentially break the decent of the keeper.  Diving on a keepers stomach only gets one hand behind the ball, exposes more of the goal, and could lead to injury. YouTube Goalkeeper TV - Soccer Goalie Diving Posture