I wouldn't go so far to say that debt is anathema to most Germans ... although statistically speaking Germany, China, and Japan are among the countries with the highest savings rate. But up until maybe 25 years ago or so it wasn't even possible as a private citizen to declare bankruptcy to allow you to start over, so you simply had to be careful with all decisions that would incur a big financial risk. Add to that two hyperinflations in the 20th century with associated money reforms that wiped out 90% of all savings (and of all debt), and you can see that while Germany is among the richest countries as far as earnings are concerned, the amount of assets for the median household however is well below average in Europe.
Anyway, the main reason for credit cards being less popular is that debit cards were introduced in the early to mid 1970s and simply took the place that credit cards have in the US as the universal cashless payment system - until the internet and web shops came along. Also, in Germany cash is still king. Most credit cards are more expensive than an EC debit card (these are usually free if you open a bank account; credit cards usually cost you a 30...40.- EUR annual fee or so), and they only fill a niche purpose (like, if you're travelling outside of Europe a lot). Which probably explains their low popularity well enough without having to dive into the elusive attempt to discover a collective psychological or cultural disposition.