Machinery needs to be lubricated to run well.

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.