Today's savviest golfers don't carry all nine irons anymore. They're using the more lofted 4 through 9 irons and replacing the others with fairway woods and wedges.
Less loft produces less backspin and requires more swing speed.
This means that you need more swing speed to get the ball airborne. Less lofted club heads produce less backspin, which means the ball is prone to slicing and hooking, and tends to keep rolling when it hits the green. Manufacturers recently increased the head size in the design of some iron sets. For most players, these larger heads are big confidence boosters. Golf players are more willing to swing harder at balls stuck in the rough or lying 150 yards from the hole because they have a larger hitting surface.