Depending on the stack, you can use some open source libraries. Datascript is quite robust, and useable from javascript/clojurescript.
https://github.com/tonsky/datascript
"An immutable in-memory database and Datalog query engine"
Most uses of GraphQL is to expose existing data from a relational database through an API, so although this looks like an interesting project, is doesn't look like it would be a contender against any GraphQL implementation.
I'd argue there are two faulty assumptions in the OP: 1. "GraphQL serves the same use case as Datalog.", and 2. "Datalog is better at that use case."
GraphQL is not a general purpose query language! There are practically no query operations at all built into the specification - it's all about field selection and parameter passing. At heart it's more like a RPC protocol geared towards requesting tree-like data. You would evaluate GraphQL against options like plain REST, OpenAPI, gRPC, not against Datatalog.
A more relevant question to ask is why Datalog never took over for SQL, but it would be harder to pretend that SQLs dominance is due to it being the current passing fad.
