From the reddit thread:
https://old.reddit.com/r/Steam/comments/ki12if/steam_now_regionblocks_all_adultonly_games_in/
https://old.reddit.com/r/Steam/comments/ki12if/steam_now_regionblocks_all_adultonly_games_in/
Translation: "Such Content is not allowed in your country"
For those not aware of German laws, pornography is of course allowed in Germany. However, a 'strong' age-verification is required by law - so that children may not access pornography. Steam's enter-date-of-birth age-verification is not considered 'strong' and as such Steam offering adult games in Germany is technically illegal.