Because it works with a credit card? Also unless you do what this service is doing by accumulating payments until a reasonable threshold is reached before actually charging the fees then even on Lightning will eat you alive. So if you have the system to handle billing this way you might as well use existing payment networks.
I think credit cards succeed because they give customers rewards, passing on the cost to the vendor and ultimately all other customers. Also because they were the only alternative to cash for the longest time