A very long time ago, the left hand was associated with the Devil, lol! True story! I think it is just one of those "it makes it specially special and scary" things about the Tarot. Don't forget, when the so-called rules of tarot were actually recorded and written down, it was still a very religious and particularly Christian kind of world, and the cards were frowned upon by the church. And therefore those who practised the tarot, not being religiously inclined themselves, maybe, may have decided it was therefore a black art...this is all just imo, but I have read similar theories.
Could also just be one of those odd traditions that accompany things throughout the ages. Like cutting the pack into three, for some schools of thought, or fanning the cards and picking at random, for other schools of thought. Just habit for habit's sake, cause the first person who wrote about it decided that was how it should be...lol!