What, I'm expected to just jump in on anything about swords here, am I?
Okay, fair enough!
The idea of all Japanese swordsmanship being right handed is basically due, as others have said, to the conformity aspect in Japan. Of course, there are always exceptions... Yagyu Shinkage Ryu, and others, have kata in their higher levels that are "left-handed", typically either for surprise, or in cases where the right hand is taken out of use, somehow. It's not dissimilar to the idea that (when wielding two swords) the Daito is in the right hand, and the Shoto is in the left, but in a number of systems, this is reversed. It all depends on usage and context.
If I was to start thinking about the placement of the sword on the left hip, therefore leading to a right-handed draw (again, there are exceptions to this as well...), I'd probably look back to armoured use, where the right side is kept back from the enemy, as that is where the gap is. If you're keeping your left side to the enemy (on horseback, as that was the dominant method of warfare), then having the sword on the right hip would leave you open when drawing it as the enemy closed. Not sure that that is the reason, but it's probably an influence.
Oh, and for the record, Sekiguchi Ryu is actually opposite to the standard power generation in swordsmanship, with the right hand providing the power, and the left hand used to stop the weapon.