Why is it better for national power to take care of coin money instead of state power

If states had access to create their own currency it would be hard to do business in between states   

bc it would be harder for people to do business

Explanation:

The framers clearly intended a national monetary system based on coin and for the power to regulate that system to rest only with the federal government. The delegates at the Constitutional convention rejected a clause that would have given Congress the authority to issue paper money. They also rejected a measure that would have specifically denied that ability to the federal government (Hammond, 92). Although the Constitution does not state that the federal government has the power to print paper currency, the Supreme Court in McCulloch vs Maryland (1819) ruled unanimously that the Second Bank of the United States and the banknotes it issued on behalf of the federal government were Constitutional.