User blog comment:Echo 1/CoL Opinions? Anyone?/@comment-4787732-20130504085448/@comment-2026417-20130504171027

The Toa Mata were the only Toa known to exist by the Matoran on Mata Nui, so naturally they'd technically be the only ones in existence. Nynrah barely traded with them, and they never would mention their Toa. Plus, most records of really anything about lands other than Mata Nui were destroyed in the "Time of War" mentioned in QftM. However, their existence and being The Six will become a major point later on.

As for the existence of other Toa, frankly, six Toa are not nearly enough to protect the entire world. But they are enough to save it. Kind of prophetic and spoiler-y there XD.