Because it utters no sound except its hissing.
Because, it appears, from the roll of swan's marks in the time of Henry VIII. that the king's swans were double marked, and had what were called two nicks, or notches. The term, in process of time, not being understood, a double animal was invented, with the name of the "Swan with Two Necks" - The Mirror, 1828.
Because of its corruption from swan-upping, or the taking-up of swans, performed annually by the swan companies, with the Lord Mayor at their head, for the purpose of marking the birds. - The Mirror, 1828.
Because it is now found in great numbers in Van Diemen's Land, in New South Wales, and on the western coast of New Holland. The latter settlement, Swan River, has been so named from the flocks of black swans on its banks.