To put an answer on this one, I believe the consensus is that it is "tradition" of the culture/style with which it originated. As this was asked on Jun 17 2015 I doubt anyone will find a more exact answer or why the tradition was to put it on the left specifically.
If anyone finds the reason this tradition was started please update with an answer, but alot of traditions started as preferences by the master and the students out of respect continued to "do things like the master" and generations later noone can remember or understand why it's done that way.