I suspect a big reason for this is that merchant agreements prevent you from directly passing on the cost of cards to customers. So if you accept Visa, MasterCard, and payment form X (where X could be cash, bitcoin, Venmo, whatever), the price you charge customers is the actual price you want to set plus the Visa/MasterCard fees, even if they pay with form X.
One way a disruptor could tackle this is by providing all (or even almost all) of those fees as cash back - giving everyone 2.5% cash back regardless of their creditworthiness / ability to get a sweet rewards card would be very popular. But then you'd probably want to not do that at merchants that aren't accepting traditional credit cards, and I'm not sure how customers would feel about this.
One way a disruptor could tackle this is by providing all (or even almost all) of those fees as cash back - giving everyone 2.5% cash back regardless of their creditworthiness / ability to get a sweet rewards card would be very popular. But then you'd probably want to not do that at merchants that aren't accepting traditional credit cards, and I'm not sure how customers would feel about this.