I wonder what the profit margin was on baking powder? Being that they could hire a team of young women to go sell the stuff door-to-door complete with cooking demonstrations.
JJ, this is how they were really making their money. 
Sure, people were baking more back then, but not all baking would use baking powder. Some baking would be yeast-leavened, and some would have used baking soda with some other acid in the recipe, rather than soda and acid pre-combined into baking powder.
Introduction of baking powder coincided with development of industrial production methods. So the cost fell dramatically, and there was a lot of margin for efficient producers.
And baking powder made kitchen baking dramatically easier. So people were prepared to pay full price.