Why is Henry IV considered the first Bourbon king of France?

Henry IV is considered the first Bourbon king of France because he was the head of the house of Bourbon and ascended to the throne in 1589 following the death of the last Valois monarch, Henry III.12

The transition occurred due to the Salic Law of Succession, which dictated that the crown pass through the male line according to primogeniture. As the legitimate male line of the Valois dynasty became extinct, the crown passed to Henry of Navarre, who was the senior member of the Bourbon line—a branch of the Capetian dynasty descended from Louis IX.23

Before his accession, Henry had been the king of Navarre and was formally recognized as the successor by the dying Henry III. His reign marked the beginning of the Bourbon dynasty's rule in France, which continued until 1792.1432