In NL cash transactions of 10k+ already needed to be reported to the authorities.

But a new law will forbid any cash transactions higher than 3k.

In Greece it is forbidden by law any cash transaction higher than 500 euros.

whats the reasoning behind the new law?

It looks like an attempt to prevent money laundering

https://nltimes.nl/2019/07/01/dutch-govt-ban-cash-payments-e...

The revenue service is unaware of cash payments/purchases made by private parties and so cannot tax such transactions. Unless those private parties volunteer the information.