Why is that? Can you provide a source? (Not distrusting you, but it would be great to have a source because I'm a writer and interested in writing about these things.)
It's the law. You can't pay in cash for anything above three thousand euros. If you're a company the limit is even lower at 1000 Euros. Interesting though that a foreigner, such as a tourist, can pay up to 10 thousand euros in cash as long it's not a business transaction.