The job was cursed.
Oh, he definitely wanted the Defence Against the Dark Arts job. The Aftermath of our little meeting proved that. You see, we have not been able to keep a Defence Against the Dark Arts professor for more than a year since I refused the post to Lord Voldemort. - Albus Dumbledore, Harry Potter and the Half-Blood Prince
It lasted From 1957 until the fall of Voldemort (1998)
1957 (Y-23) [Voldemort] resurfaces and applies to teach [Defence Against the Dark Arts] Ten years after the death of [Hepzibah Smith], [Voldemort] returns to [Hogwarts], where Dumbledore] refuses to grant him a teaching job. He then curses the [Defence Against the Dark Arts] position, and since that day, no professor has stayed for more than one year in that post. Again we don't know exactly when this happened, but since most evidence points to [Dumbledore] becoming headmaster in 1956 [[Y-24]], [Voldemort] couldn't have come looking for a job much sooner than this. (HBP20)
No one could hold that position for more than one year
It's well seen throughout the series that there is a "curse" on the DADA teacher position, that no one can hold that post for more than a year
Is there (preferably canon) information on:
- How did Voldemort manage to do such a powerful curse?
- Why didn't Dumbledore, knowing about the curse, not do anything to lift it?
- Why didn't Voldemort put that curse in the headmaster's position, or on the whole Gryffindor house?