If you keep the existing rate structure, payment processing represents a ~3% sales tax that yields a huge margin for all players, a tantalizing revenue stream to nationalize.
Id you use the state resources to subsidize the price down to zero, you have a great "pro-business"/"pro-inclusive-economy" plank for political points.
Either way, you get a huge amount of data and a massive lever you can use for political ends. Think of the deplatforming debates of a year or two ago, but magnified by "this is basically the only payment platform used in the country."