structuring financial transactions to evade currency transaction reporting requirements is a violation of federal law.
If you own a business that deals in a lot of cash and gross under $10k a day to the tune off $8500 one day, $9800 another, etc. you may find yourself in trouble for the appearance of evading currency transaction reporting requirements.
Add to this the fact that when this was enacted you could have bought MULTIPLE new cars for under that amount of money you see the challenges inflation has brought to the issue.
Try buying a car with cash.
An old farmer I know did, and the cops showed up a few minutes later. He had bought cars with cash for decades.
It is now illegal to use cash, or not finance large purchases.