I'll try to explain why.
The setting was really promising and for once it wasn't the typical monster-slaying Anime, but since they revealed so much in such a short amount of episodes, it got boring real quick, and by the time you find out the main protagonist is, in true Anime/Manga fashion, virtually invincible, you can't enjoy much of it.
I also find the Anime to be really colorless. I know it's supposed to settle the fact that it's a dark world in despair, and also medieval-ish in setting, but there's also lush greenery outside the borders of the cities, so those should at least look more lively to have some contrast with the depressive innards of the human-built cities.
Then there's also the whole antisemitism and Nazism ideologies stuff but that's another can of worms.
