First: Do the rules specify that Heavy weapons are two-handed? I may have just missed that.
Sure, great clubs and two handed swords are two-handed in real-world use... ...but what about long spears?
I mean the reason the spear had a place on the battlefield for thousands of years: you can still hold a shield and make effective spear attacks with a very long spear. Unlike a polearm, you are only stabbing the user doesn't need the leverage required for a swinging weapon.