Some guesses, I have no skin in this game:

- historical inertia, i.e., SQL being better known, being taught to a lot more people, already used everywhere; even if every new project started Datalog, we would "still have SQL databases" for many decades, see the recent publicity for Cobol

- aggregate functions, sorting of results

- syntax: "select foo, bar from table_with_many_many_columns" is easier to get right than "table_with_many_many_columns(_, _, _, _, Bar, _, _, _, _, Foo, _, _, _)" (though in most other aspects Datalog syntax is indeed superior)

Take a look at datomic, it uses datalog.

Seems awesome from the outset. I especially like "Datomic stores all history, and lets you query against any point in time."