That's because if you use your left hand for OH, you still have the normal access to RU turning, and you still have a somewhat normal grip on the cube. It definitely seems counterintuitive at first, but it makes sense.
If you want to use right you can be equally fast, but it is at a disadvantage because you will have to learn righty algs.
My friend loves learning algs so he uses righty and avgs somewhere around 20 for OH
Most people use right handed algs. It's easier to do right handed algs with your left hand, because your fingers are on the right side of your left hand.