That's a very justified question!

In my opinion, Datalog or — more generally — Prolog is where many querying APIs will eventually arrive, because this syntax has many nice advantages: It is very convenient, expressive and readable, programs and queries can be easily parsed and analyzed with Prolog's built-in mechanisms, there is an ISO standard for it etc.

When semantic web formalisms were discussed, Prolog was sometimes mentioned. However, it is so far not very widely used as a querying or modeling language even for semantic web applications. Recent advances in Prolog implementations may increase the uptake of this syntax.

Also, public perception of Datalog and Prolog lags somewhat behind recent developments in these areas and what these technologies can actually do or what they even are. For example, here on HN, many posted articles that claim to be about Datalog often show snippets and examples that use completely different notations which are no longer a subset of ISO Prolog and therefore also not Datalog. Thus, they also do not immediately benefit from recent advances in Prolog systems in that they cannot directly be parsed and interpreted by them. It may take some time to advertise these improvements, and increase interest in these formalisms.