Possible reasons why achievements could be unattainable are:
[x] Because the difficulty setting doesn't allow it. (Oversoul achievements cannot be attained in easy mode, and finish game in Hard mode is an obvious reason why the easy mode doesn't allow it).
[x] Achievements only given in Game Jolt Only dungeons are naturally unattainable when you are not logged in on Game Jolt
[x] Some achievements can only be attained during the New Game+
[x] And there's of course "The True Abyss" in which many achievements of the normal game are unattainable, but there are also trophies which are available in the True Abyss only.
Possible reasons why achievements could be unattainable are: