Changing it would mean they have to fight almost the same way with an injured leg as they do unhurt without special provisions. The truth is, it is harder to keep fighting when you have to remember a leg is wounded and keep it stiff or drag it. By having to remember the injured limb and having to drag it or just keep it stiff you really are distracted and fighting at a disadvantage, not just mimicking something. Because someone is still standing and fighting with a wounded leg, if they make a lunge or step out and expose a leg to another hit, they know they will die. This is a big difference from the supposedly awkward position kneeling down was originally intended to cause.
Assumptions create ramifications far down the road that are usually quite subtle.
This is the case with kneeling down requirements during any sparring that is simulating any manner of Medieval combat. Many participants refuse to re-consider issues of rules. These are all reasons for lack of examination of the rule-structure or willingness to implement change. If one finds themselves growing angry while reading this critique of kneeling down rules, perhaps they should asked themselves why that is, and engage in some introspection about their own preconceptions. Just try out this alternative for a decent length of time.