Because for some wierd reason we have more people in prison than just about any other developed country. What the prisons are like is a small piece of the puzzle but only a small piece. I would bet that we have two or three times as many people in prison, per capita, as Germany and most other nations.
Sorry, I still don't see you making any connection between the relative rates of incarceration of our two countries and a rating system for German prisons.
We probably do have a higher per-capita rate (I don't know for certain). Even if we do, what does that have to do with whether or not there's a rating system for German prisons?