Everyone welcomed the currency issued by the bank; it was attractive, easy to carry, and its value was about the same. Hence, it was quickly accepted by everyone.
In fact, even if they didn't accept it, there was no other choice. As long as you were in a city governed by the Sky-opening Sect, all transactions had to be conducted using the bank's currency, or else you simply couldn't trade in these cities. If someone was caught dealing under the table, not only would that person be executed, but all of their property would be confiscated! The punishment was exceedingly harsh!
Such regulations would be very difficult to implement in a city governed by other forces, but these cities were the Sky-opening Sect's own territory. If you're dissatisfied, you're free to leave; no one will keep you. But if you choose to stay, you must abide by the rules. Resistance is allowed, but all rebels will be executed without exception!