I think the logic behind it is making a blade heavier doesn't really make it any sharper, which is the main reason why blades exist*, for stabbing and cutting. Making a weapon heavier means you can apply more force as the weight of the weapon is added to your attack, but that's not really beneficial to a sword because you're just wasting additional energy to do the exact same thing, cut or stab, which can be done just as effectively by using a sharper weapon. (that's just how pressure works.) Where-as with a mace or hammer, the idea is just to project a shit-ton of physical trauma into a wide area of the opponent's body, to shatter bones and cause massive internal damage. This is directly proportional to physical force and weight. (which incidentally is why it would be my preferred weapon, because I'm big and slow.) 

*disclaimer, I'm aware blades aren't anything like what you see in bad samurai movies. In fact, it was counter-intuitive to make your blade -too- sharp, especially with longswords etc.