They are looking to get tax revenue from the working for cash underground economy. Estimates have that as exceeding 2 trillion dollars of unreported income.
https://www.investopedia.com/articles/markets/032916/how-big-underground-economy-america.asp
One should also ask...why they are suddenly concerned about it beyond taxes? Are they planning on doing something that will cause millions of people to take their cash...essentially ‘off grid?’
There is more behind this than what people are perceiving. The banks complaining about the burden of reporting is just window dressing for now. They really don’t have a concern when the real train heading down the tracks is the new digital currency.
Suddenly it makes sense to question people who even have $600 in cash or why they need to take out $600 in cash. People will avoid using cash and thus giving birth to the digital dollar.
This is to make it a hassle to use cash. This is part of the propaganda to get people ready to accept the digital dollar.