"Next business day" is a huge amount of time relative to the "instantly" that the value changes hands in a cash transaction. Cash also eliminates any need for prying into the background of the buyer: Their proof of ability to pay and payment itself is the stack of cash they have in their hand.

If the government wants to eliminate the requirement of accepting large amounts of cash, I can see that. If you're not equipped to handle being paid in a large cash sum, it can definitely be inconvenient. But making the exchange of large sums of cash between two consenting parties illegal is bold-faced tyranny.