There could be many reasons for swans not inhabiting certain lakes. Pollution, lack of food or nesting resources, presence of predators, ponds or lakes off the beaten path and not familiar to the swans on certain migratory routes, the presence of development, human presence or domestic animals such as dogs and cats.

Another major reason would be the lack of wild swans in your area. If wild swans are not present, you may be seeing captive swans in lakes near you. The Regal Swan