Because Mata Nui left Toa eventualy become extinct and become legends. The future matoran and agori build "Heros" based on the legends of the toa.

One day a future matoran named Von Nebula, who is exploring the ruins of the MU finds the Antidermis pool where Makuta were made,he touches it and is corrupted and mutated by it. He learns of Teridax's plan and of his defeat and vows revenge against the decedents of the matoran.

Not really, but it would be cool.

Also,

Inspired by this topic right here.