I believe this goes all the way back to horse-mounting - mounting from the left is easier, as your primary hand weapon (sword, mace, club, whatever) is likely t

I believe this goes all the way back to horse-mounting - mounting from the left is easier, as your primary hand weapon (sword, mace, club, whatever) is likely to hang at your left side, where you can easily reach it with your right hand; this makes mounting from the right considerably harder, as the weapon gets betwween . (It would probably reverse for left-handed fighters; this had very little impact on today's bike mounting and kickstands, however)