"Why do we still use SQL" and "Why do we still use relational databases" are two very different questions. They seem like much the same thing, because SQL is pretty much the only query language offered by relational database systems nowadays... so if you use SQL, you use an RDBMS, and vice versa. But other query languages used to exist. There was QUEL [1], for example. It seems to have fallen by the wayside; most people have probably never heard of it. I guess there is very little room for multiple languages in this particular space.