Givens and Reasons

Often my friend Tennis invites me to join him for the private sword class he takes with Sensei Mike Esmialzadeh. We practice the partner exercises from the system and I get to be 'the opponent' so Sensei Mike E can troubleshoot. Class is 6.15a.m. and without doubt is one of the funnest ways to start the day.
Sensei Mike E is high ranking in many Japanese systems, but has also trained various Chinese Arts, and Eskrima under Mike Inay. He is hugely knowledgeable, well read and highly skilled, and with this varied background can see behind the Arts to the principles that lie beneath - he can explain them, and he can do them.
Like I said - big fun.

Last class we were working attacks from the draw - counter ambush type stuff from the Toyama Ryu Battodo system, and that led to a discussion of the various ways that swords have been carried in different cultures and over time.
Mike pointed out that pre 1570 Katanas were generally carried edge down (as is the norm for sword carry in many cultures), but after 1570 though this was still true on the battle field, on the street the blade started to be carried edge up.

The 16th century had seen constant civil war in Japan which was to continue into the early 1600s, but after 1570 the climate quietened down, and the result was a country filled with warriors who had known nothing but war for generations, all carrying swords into an era of peace.
As one could also imagine, old feuds from the war era bled over into these times, and the integration of past enemies into peaceful neighbors was not a smooth process. This resulted in constant confrontations, street brawls, ambushes and assassinations. An attack could come at any time, from any direction, front, behind, round a corner, and could be a cut from above, the side, a poke, a disarm, and there could be more than one of them.
It became important to have counter ambush skills and a quick draw.

So why did this influence the way the sword was carried?

Carrying edge down gives you one good angle to draw and cut from (#3 in TRB), but makes it harder to draw the blade upwards or horizontally as you have to twist in and up on the scabbard to change the angle, and your wrist does not have so much range of motion in this direction - remember the sword is curved and so is the scabbard. But keeping the edge up means it is much easier to angle the body or twist the scabbard out and down (a much more natural movement) to change it's orientation, giving 5 good, quick draw directions, enough to (potentially) take care of any attack angle, from any direction when combined with movement.

So even in the most traditional systems, "why?" questions have answers rooted in pragmatism, historical or otherwise ... or should do.  
Everything should have a good reason behind it, and asking "Why?" and discovering these reasons is a crucial part of understanding.