This achievement unlocks after you've unlocked all other achievements.
This achievement has a higher distribution rate among players than the Masochist achievement, which doesn't make sense as it shouldn't be possible to have this without having all the others as well. However, while you do need the Masochist achievement to unlock the Insanity achievement as well, the Masochist achievement was added later than the rest. So before it was added, people could unlock the Insanity achievement without having unlocked the Masochist achievement, as it simply didn't exist at that point.