I always found that so weird! The Exportweltmeister, producing some of the most advanced equipment... and in many places you can't pay with a card at all. Why do you think it is?
For many small businesses non-tax registered money is the lub which makes them run well.
Jokes aside the price of getting a card terminal where for many businesses completely unattractive for a long time and often still are if put in context to the number of people which will use it.
I know one local takeaway which stopped accepting card payment after their terminal broke recently, as it wasn't worth it to buy a new one. Instead they now allow sending money by PayPal, but non-advertised and mainly for a single specific big recurring customer and sometimes if someone doesn't has cash with them.
We Germans seem to love our cold hard cash, so the incentive to get a card reader is lower. There are even automatic coin counting machines in some self-checkout desks...
