The buttermilk is more acidic than regular milk, which will throw off the baking powder. That's my theory, at least.
This is from Baking911. I think it answers the question in the inverse.
"It's best to use whatever the baking mix calls for. The leaveners in the recipe are formulated to activate based upon the pH of the ingredients. If the boxed mix said to add buttermilk, the batter's leaveners are formulated to counteract its acidity. The baking soda in the mix activates from the acid, provides carbon dioxide for leavening and at the same time neutralizes the flavor of the acidity in the batter giving you a nice taste and texture (too much acid in a batter would give you a craggy and uneven texture). If you added milk instead, the recipe would taste almost bitter or salty from the unneutralized baking soda in it unless it had baking powder instead."
Less is not more. More is more and more is fabulous.