The moat
Two-sided network (cardholders and merchants), brand at the point of sale, and regulatory/compliance burden that freezes the oligopoly with Mastercard. Switching costs for issuers are high; for consumers they are a wallet habit.
Pain, risk, or retraining required to leave.
The product gets better as more people use it.
Patents, data, licenses, process knowledge, regulation.
Fixed costs spread over a volume rivals cannot match.
Structural ability to be the low-cost producer.
Pricing power from trust, habit, or identity.