@cryum
I guess because it's not very chivalrous to try and trip your enemy. Combat in this kind of duel is supposed to be about weapon technique and overpowering or outskilling enemy and striking their body with weapon, not trying to knock them down/over.
It's how in boxing you can't hit below the belt or kick, or how you can't pull someone's shirt in soccer to knock them over or keep away from ball physically, but accidental bodily contact is not always a foul.