I can swap out Angular for React + Redux in my application without my backend knowing about it. The same is not true of React + Relay + GraphQL.
It's a bigger, more involved decision that doesn't strike me as being particularly practical for an individual or a small team. Couple that with the fact that making a bigger, more involved decision is harder for bigger teams and it becomes apparent why it would be hard for Relay + GraphQL to "win" over Redux.
I can swap out Angular for React + Redux in my application without my backend knowing about it. The same is not true of React + Relay + GraphQL.
It's a bigger, more involved decision that doesn't strike me as being particularly practical for an individual or a small team. Couple that with the fact that making a bigger, more involved decision is harder for bigger teams and it becomes apparent why it would be hard for Relay + GraphQL to "win" over Redux.