Why was the Mahdi named the Mahdi?

"Mahdi" is not his given name; rather, it is a title for him. His name is Muhammad ibn Abdullah, which matches the name of the Prophet, peace and blessings be upon him, and his father's name. "Al-Mahdi" is a title for him because Allah Almighty guides him and rectifies his affairs in a single night, meaning Allah accepts his repentance, grants him success, inspires him, and guides him.