When you're able to establish physical play with a big hit or whatever, then it breaks another team's momentum. When you're not skating, you're trying to playing physical, you're in-between all the time. In-between also means dividing attention between skating with the puck and watching for the hitter and unloading the puck as quickly as possible. That is no way to survive in the playoffs. As in every major sport, hockey players have become bigger, stronger and faster.
Basic physics says that means big hitters have become more punishing.
It's not a good combination. Sizes have changed dramatically in recent decades. The New York Islanders won four Stanley Cup titles in the 1980s. Defenseman Denis Potvin was their big hitter. He played at 205 pounds.