heavy armors were actually VERY hard and cumbersome to move and fight in. it required both high strength and high endurance. not to mention falling in one of these made it near impossible to get up no matter how strong you were. this was why everyone wasnt running around in a set of full plate,not to mention how expensive it was on top of that.
Whoa... no not even close. A full suit of armor in the late medieval period weighed about 50-60 lbs (I think you could stretch it to 75 lbs if it was really thick). My load out on my first deployment averaged 75-80 lbs on any given missions (there is a reason why we drive/ride when we can). Even when I deployed later with an M-4 instead of the 16, it only dropped my given weight by a few pounds. Now, that's just a normal grunt's weight, take in an account for the medic or anyone who has to lug a MG around. I think we were told that some loadouts can exceed 120 lbs.
A knight had their weight completely distributed around them as well. They were fortunate in that believe me. Rather than having your weight around your waist, chest and head. Not to mention that they had squires and only wore the armor before battle or when they were in enemy territory. Most of the time when traveling they wore lighter armor that was more akin to what everyone else wore. They also didn't have to carry any of the rations because they had squires and baggage trains. So aside from weapons and armor, that is all they had to wear.
Also, the myth behind not being able to get up after falling is totally farce. I've seen people get up in 65lbs of full plate armor. They said it wasn't nearly as difficult as they expected, but kind of awkward at best (I didn't get to try because I was too short to wear either one). Granted, if you were to fall during a battle and are already bruised and tired, I could see this becoming an issue. But that would be true about anyone.
Also, I believe the reason why Fredrik I drowned on his way to the crusade was not solely due to the weight of the armor. If I remember correctly, the river's waters were a bit higher and the current was a bit stronger than usual. It only takes about 6" of water to knock a man off their feet if it is flowing hard enough. So, after his horse freaked, and he fell in, it was probably impossible for him or his men to get any hold before he drowned.
EDIT: Accidentally quoted the wrong guy at first