Sometime the best way to why a (Chesterton's) fence is blocking the road is... to remove it and see what happens!
Sorry, not really an answer to your problem. But I feel you, this is a genuinely hard problem.
Keep in mind that, pretty often, the reason something is the way it is comes down to "no real reason", "that seemed easier at the time" or "we didnt know better". At least if you don't work on critical systems.