The reason for non-acceptance or limited acceptance of credit cards in german stores (in particular supermarkets - but not limited to) is simply because there´s a huge difference in the commission the dealer has to pay to the CC company compared to the debit card system.
CC companies take about 2-3% off the total but the debit card system charges a "flat rate" of less than 50 Cents regardless of the total amount ...
We have similar commission rates in the UK (i.e., percentage based for credit cards, a flat fee for debit cards), yet supermarkets here do take credit cards, and have done for at least a couple of decades (I am old enough to remember a time when hotels, restaurants and high end shops took them but supermarkets didn't)
I still think the issue is one of lack of demand. Germany was quite a late starter when it comes to credit cards. I still remember the total amazement shown by German friends when I had a credit card, as a university student, in the late 1970s. In Germany credit cards were for the rich and for frequent business travellers (and came with quite a high annual fee - whereas they were free here). Credit card use is now much more common in Germany - but it's still lagging behind other countries. I would say that Germany is now where the UK was in the 1980s.