Assume there are 40 feet between infielders. Further, assume an infielder can react laterally 10 feet on a 50 mph ball, and only 5 feet on a 100 mph ball (because it travels through the infield much quicker). Therefore, on a ball hit 50 mph, there is a 20 ft wide area the infielders can't reach; but on a ball hit 100 mph there is a 30 ft wide area. That gives the ball a significantly better chance to get through the infield when hit hard as opposed to not. Exit velocity is very important, although as Jeff said, if you are sacrificing contact for exit velocity, it can quickly turn into a negative.