Because while he was on earth, the people whom he ruled over where not all literate, most of them were slaves. So we only know of him through paintings and the monuments made for him.

We actually have pretty good evidence of this, through the myths and legends of ancient Egypt. Unfortunately these have been corrupted by time and human agency (cf. the "Avatar Theory" by Hogan), so what evidence we do have of his rule must be sifted from the detritus of myth. This is quite similar to the distortion of the Asgard by the Scandinavians during the Migration Era.