I never disputed them being able to run, not at the same speed but still able to happily run. You could tell, however, that he still doesn't run entirely normally, but that is not the dispute I am making

Ever seen an un-armoured person fight? The knights there were not as dexterous, most obviously you saw him stumble as he was hit, and he was slow to turn back. When at the very start one of them strikes fully with his poleaxe it takes him a good second to wind back to ready. 
Notice their large, exaggerated movements? That is not a normal fighting style. Their fighting style is based on the idea of counter attacking, because without their deftness they are unable to quickly respond by jumping back into a guard, as they would most likely lose balance on the uneven terrain and fall over due to the extra weight. As both combatants realise this it becomes much more intimate, relying on grapples to bait their opponent out into an attack, over extending and countering. This last point about grappling however could be attributed to their use of long weapons as well, but no historian worth their salt believes a single factor is important above all others