Why did Frederick William have to give up his conquests in western Pomerania after the First Northern War?

Frederick William was forced to relinquish his conquests in western Pomerania following the First Northern War due to French intervention.1

Although the Elector had successfully driven the Swedes from the region with the support of his allies, Poland and the Habsburg emperor, France intervened to protect its ally, Sweden. This diplomatic and military pressure compelled Frederick William to return the territory. The formal renunciation was ratified in the Treaty of Oliva in 1660, though in exchange, he secured confirmation of his full sovereignty over the Duchy of Prussia.1