The delegates at the Constitutional Convention decided not to amend the Articles of Confederation because they believed that the current structure of the government under the Articles was too weak to effectively address the needs of the newly formed country. The Articles of Confederation provided for a loose confederation of states with limited power given to the central government, which was unable to regulate trade, enforce laws, or raise revenue effectively.
The delegates saw the need for a stronger central government with the power to regulate commerce, maintain national security, and promote the general welfare of the country. They believed that a completely new document, the Constitution, was necessary in order to create a more effective and efficient government. This new document would give the central government more power and authority to govern, while also preserving the rights and liberties of the individual states and the people.
In short, the delegates at the Constitutional Convention chose to create an entirely new document because they believed that amending the Articles of Confederation would not be sufficient to address the needs and challenges facing the country and that a stronger, more centralized government was necessary to ensure the success and stability of the nation.