Why does it need to say “explicitly”?
It is reference to Article II of Confederation:
“Each state retains its sovereignty, freedom, and independence, and every Power, Jurisdiction, and right, which is not by this confederation expressly delegated to the United States, in Congress assembled.”
The anti-federalists demanded insertion of this Article as an amendment to the Constitution.
Durus wrote: ‘Why does it need to say “explicitly”?’
I agree that it doesn’t really need to to be effective. However, if it did, it would limit federal power to an even greater extent.