If the cygnet was the last to hatch, i.e., the runt (and probably female because the male parent is allowing it to remain), it may not have fully developed wings in time for migration. If the cygnet has something wrong so that it has difficulty or cannot fly, then this would also be a reason why it has not departed. If it stays (or is allowed to stay) during the upcoming nesting season or next migratory season without trying to leave or find a mate, then there is probably something wrong and it cannot fly. The Regal Swan