Ultimately GraphQL is popular because the query ergonomics fit well with the component architecture everyone is using these days, and once you're familiar with it you can be extremely productive.
Were they using that architecture prior to moving to GraphQL, or did the tail wag the dog in most cases?
GraphQL works well with declarative UIs, because you can define fragments and combine them to create queries that best match the data the current set of components you are loading require. In this way, it's flexible in a way REST isn't (at least typically) and syncs well with Declarative UIs.
I can't speak to datalog though.