We addressed this in a recent Kickstarter post here, if you want the long explanation (scroll down to the section labeled "Clarifying the situation with the Steam release's age rating").
The short explanation is that despite the fact that we did NOT intend for Idol Manager to be a "adults only" game, Steam decided last week to classify it as "adults only," which prevents it from appearing in certain territories, including Germany. Since then, we have been working with Steam to attempt to rectify the situation and submitted a new build of the game, but unfortunately because this came up so recently, we were not able to resolve it in time for release. We are still waiting for their verdict, and due to the current uncertain situation, we can't disclose any further details, but once Steam has passed judgment and re-evaluated our game, we'll have more announcements/updates to share.