Why can I not buy iBooks in German anymore?

iBooks Store like all other stores, you can only buy what's available in your country iBooks store.

The content provider is the one who determined where Apple can sell their content. (Hint: Copyright)

If you have a certain book in mine, write to them about it.

When you can't find something in the iBookstore, it is often best to try one of the other stores that also offer free ereader apps, like Amazon, Kobo, Nook, Google, Sony.