The stopper (or at least one) is probably credit card transaction fees. There is usually a minimum fee of like 15¢ which makes it less feasible to do transactions under a dollar.
Obviously that’s very expensive compared to a subscription but that’s kind of the point.
FedNow should help in the longer term as non-credit card payment methods show up that use it, but it's still not going to get to the realm of 10¢ micropayments (the service has a flat $0.045 per transaction fee, and companies still have to then consider their own service cost overheads on top of that).