Because the Triforce only expands the range of abilities its holder already possesses - in this case, it expanded Ganondorf's magical powers, enough to make him immortal; thus, the Triforce didn't give him immortality directly - he cast a spell or something using its power to grant himself eternal life.
...Or it's because the Golden Goddesses aren't good at planning.
I don't believe he was given immortality per se, he just has an extremely long life. Since Gerudo men are only born every 100 years, it is plausible that Ganondorf will live to be at least 99. The first response does have the right of it though, when one is given a essence of the Triforce, that person's abilities are increased dramatically. Ganondorf's magical power in its original state was powerful enough to bring distraught to all the Guardians of each tribe, making his strength and magical power abilities boost high to God-like levels after obtaining the Triforce. It is therefore plausible that Ganondorf's life was extended as well. Timelinesplitter 07:37, May 15, 2012 (UTC)