I’ll try this one last time. The government of the United States is a government of limited powers, not inherent powers.
Example:
Article I, Section I:
Section 1. All legislative powers herein granted shall be vested in a Congress of the United States
Note that the words “herein granted” are to make it clear that Congress does not have plenary power.
Just to avoid any mistake, the Constitution was even amended immediately to clarify this point, and overcome objections that someday, people like you would misinterpret it:
Amendment X
The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people.
Everyone involved knew what no one today seems to remember, that a national government of inherent and plenary authority is utterly incompatible with a Free Republic.
So when you ask “why did the Constitution only forbid states from paying with paper money”, you are asking a question that misinterprets the whole design of the Constitution. From their perspective, only the States could possibly have some power that needed to be forbidden; the new national government only had those powers which were expressly granted. And they had just struck down a proposal to grant such power, leaving Congress with only the authority to borrow money, and coin money. You cannot “coin” paper.
The States did have the power to issue paper money until they relinqished it by contract, so to speak, in the Constitution. But they never granted it to Congress, by ratification of the Constitution or otherwise.
I agree. Here's one of their limited powers, Congress has the power: To coin money, regulate the value thereof
I’ll try this one last time. The dollar (money) was originally set at 27 grains of gold. If the Congress has the power to regulate the value of money, they can change the dollar to 13.5 grains or 5 grains or even zero grains of gold per dollar.
Now unless you have another passage in the Constitution where it says they can't value the dollar at zero grains of gold, I suspect we're done here.