I think that Redux provides two things that people like:
1. A pattern to organize their state in that was testable and repeatable
2. A way to maintain global app state without prop drilling
I think that (1) is actually the reason that many people pick Redux in the first place. The amount of global, app-wide state in /most/ applications is fairly small IME. I think that the ReasonML team (which works semi-closely with the React team) is banking on things like Apollo, "suspense", the new context API and other things to further reduce the need for a global state store like Redux.
The ReasonReact devs are trying to follow the principal of least power in providing something that solves (1) for component-level state, which solves the 80%, and wait for the rest of the ecosystem to shake out for the rest.
