It's kind of random. There were plenty of competing languages in use until the 90s. Even our beloved Postgres used QUEL for its first decade of life. But Oracle took the dominant lead and its competition either disappeared or started adding SQL support to be compatible with it.

Had a different butterfly flapped its wings all those years ago, things could have turned out very different.

> SQL is basically math.

All programming languages are basically math.

> but the principles behind it are set theory

Thing is, one of the challenges with SQL is that it doesn't adhere to set theory. This leads to some unintuitive traps that could have been avoided had it carefully stuck to the relational algebra.

Such deviation may have been a reasonable tradeoff to help implementations, perhaps, but from a pure language perspective it does make it more terrible than it should have been.