It’s unavailable because Steam doesn’t want to set up proper age verification.
https://www.theouterhaven.net/2020/12/steam-region-blocks-adult-only-games-in-germany/
All adult-only games have this issue, it’s Valve’s (Steam owner) decision and I can’t do anything about that.
What I know, you can ask a friend from another country to purchase it as a Steam gift for you or use VPN.