That's a great question.
I think it's because of, well, a few things. Just because interchange fees are changing doesn't mean the overall pricing model of any given payment processor has to change. That's because the market for payment processing, especially credit cards, is fairly competitive and fairly well developed. There's a lot of differentiation in the market. Firms don't just compete with each other on the basis of price. They also compete with each other on the basis of value added.