It might have a lot to do with our advancements in metal alloys these days. Plate armor, as I have read, could weight between 50 and 100 lbs. I'd like to see cartwheels in that. The 100lbs may have been for specialized jousting or ceremonial armour. And 50 is easy to carry around when distributed over your body. Average full-plate weighed in at 35-45 lbs. Consider this: a lot of people carry 45 pounds of excess fat on their bodies. I know some of them and they don't have a problem getting up off the floor. Some can do cartwheels and other assorted acrobatics as well. It has nothing to do with modern metallurgy. The people who originally made the armour back in the day were just as good with metal as smiths these days, and the smiths these days who are making armour, if they are going for accuracy, use the same methods as those in the Middle Ages or early Rennaisance. What people also forget is that a Medieval knight was a highly-trained professional warrior who had, from a young age, been trained to fight, move and perform acrobatics with armour on. They had to. The images of knights foundering around helpless on the ground and unable to get up are complete myths.