SCADA networks historically have very low available bandwidth (often due to being on the other end of a 10 mile radio link over 915 MHz with the radio & RTU powered by solar panels) and rely on a poll/response methodology. Even if nothing has changed, the poll happens, and the response sends the same exact data every poll. MQTT can be a far more efficient protocol if you only send updates. Multiply this situation by 1000x RTUs (now imagine you have a slightly better radio link from office to a remote tower, but that remote tower is communicating with 10x RTUs). You can see the need for efficient communication.

Now - realize this is how natural gas pipelines are monitored/controlled in the mountains. You want that kind of communication to be reliable and scalable right?