There had to be something gravely wrong with the cygnet, i.e., injury or illness (which could explain why it was in an abnormal position--floating on its back). The parents knew something was wrong which is why they did not go to the youngster.

If there was something wrong such as illness, there are no guarantees that whatever was done, even treatment by a veterinarian that the cygnet would have survived. There are also other factors which include: the age of the cygnet, how long it was in the water and upside down and how much water was inhaled or ingested. The length of time in the water and the amount of water found in the lungs and body can affect the body's function hours after rescue so drowning could have also been a factor in the cygnets death.

No matter, you did your best and again, there are no guarantees that anything further could have been done for the cygnet. Situations like this are difficult. Thank you for rescuing him and trying to do something. The Regal Swan