Remember what the Wikipedia article said? Well-made medieval armor weighs LESS THAN MODERN COMBAT ARMOR.
If you had checked the source of the claim on wikipedia you might have noted that page 119 of "Warrior Race: A History of the British at War " says nothing about the weight of a suit of armor, rather it talks about the weight of a lance. Further more, page 122 of the same book gives the weight as 60lbs. Page 151 of "The art of war in the Western world" by Archer Jones giuves the weight of a suit of plate armor at 70lbs. I've seen sources (Not energetic enough to dig the books out of the garage) that gave full suits of jousting armor at over 100lbs, Jousting armor was useless in a real fight (among other things the helmet was bolted to the brestplate so you couldn't move your head, but you also didn't have to worry about your neck being broken) but it's probably the source of Mark Twain's often repeated "knight loaded onto a horse by a crane" idea.
This, too, is the more common figure I've heard.
Soldiers throughout time have worn about 60-80lbs of gear, from Roman legionaries to medieval knights even to soldiers today.
Why? Because a soldier needs a lot of stuff to do his job. Armor, weapons, supplies, tools, and a backup in case a weapon breaks. A person can really only carry about that much weight before they're too slowed down by the weight to be effective. If they can, they'll set their pack down before going into battle, but only if the battle is not mobile at all, such as they're defending a position, but thats still risky because if they have to move and can't pick up their supplies again, they're probably screwed at that point.
Used to be most of that weight was in the armor and weapons, and squires hauled around the other supplies. These days the armor is much lighter, but soldiers now carry their own supplies on their backs.
Take a look at any picture of a US soldier, such as in Iraq or Afganistan. They carry around a ton of stuff in addition to their rifle.