Why did the cardinals build a nest and not use it?

Cardinals may build a nest but not use it for several reasons, including environmental disturbances, changes in food availability, or the presence of predators. They might also abandon a nest if they sense it's unsafe or if the timing for breeding isn't optimal. Additionally, some pairs may build multiple nests before settling on one for laying eggs. Ultimately, it's part of their natural behavior to ensure the best chance of successful reproduction.