The main reason of its not being used is the fact that it was not released for production (until today). Consensus in client-side validated systems (RGB belongs to them) is kind of ossified from the day one when you start using in production; it is much harder to change that than blockchain consensus. Thus, it took many years to build a system which has everything needed, including zk-STARK readiness.

I am quite sure we will see a boost in RGB adoption over the next months after the release.