Current message based payment systems don't address some common IoT use cases.
Scale / Lightweight Processing
IoT payments must be high speed and light-weight to process. This is a bigger problem that it first presents (e.g. Visa.Net advertises the ability to scale to 65,000 payments per second)
Fee structure tends to drive a lot of the adoption behavior in payments sytems..
e.g. Most "real-time" payment types are expensive... RTP is priced to be expensive, still waiting for FedNow. None of them are designed to handle micro-payments. Crypto currency requires proof of stake etc which are inherently expensive.
IoT devices tend to be low power, embedded devices which are often disposable by design. The ability to initiate payments needs to be fault tolerant and self healing and needs to be (disposable) device independent.
IoT devices may be tasked to operate at "line speed" e.g. to generate "click" based billing charges.