The German market is very small, the English market very large. This means that if you translate from German to English, you invest a substantial amount of money to get a book onto an already crowded market dominated by English authors. That's a huge risk for the publisher. If you cannot market the book well enough on the foreign market, you will loose money. Literary translations are not cheap.
The other way around, if you translate a book from English to German, you can estimate very well from the performance on the gigantic English book market how it will sell in Germany. Bonus points if the author is already known in Germany anyway. The risk is lower than opening a program slot for a new German author (who would require additional marketing), and the cost of the translation is offset by the reduced risk.
At least that's my explanation. Maybe I'm wrong about it, I'm not a publisher.