to me, a big reason for the left foot is that when you take that next step, (if you hit the bag with your right foot), that next step would be with you left and you're very likely to slip or slide (especially with a wet track). And, as Jerry said, that left foot would be in a bad position (outside the body) on the next step. Can't really dig in with the left foot, but you could with the right.
So, for me, its more about that next step that the step that hits the bag.