Because it was the symbol of the overflowing of the Nile, from its arrival, breeding season, and departure, coinciding with the commencement, etc. of the fertilizing inundation of that river.
It has been frequently represented on their hiero-glyphical monuments, and prepared, like the human bodies, in the form of mummies, great numbers being placed in certain vaults. At present it is rather uncommon, at least in Lower Egypt. - Blumenbach.