There are several possibilities. The first possibility is that there was something wrong with the cygnet initially and the parents abandoned it.

The second possibility is that this cygnet was not that swan pair's cygnet. This would certainly get a straying cygnet killed.

The third possibility is that the cygnet was away from its parents more than 2-3 hours and they gave up on it. Once separation from their parents is for an extended amount of time, then the parents will usually not take it back. Unfortunately, this is nature. The Regal Swan