The Constitution was the second charter of govt of the US. Why did it replace the Articles of Confed?
The Articles of Confederation were written to create a confederation, a loose association of independent states. Having just fought a war to rid themselves of the perceived tyranny of the British King George and Parliament, the former colonies did not want to yield any more power than was essential to any form of centralized government. Therefore, the Articles of Confederation specifically stated that
Each state retains its sovereignty, freedom, and independence, and every power, jurisdiction, and right, which is not by this Confederation expressly delegated
and established that the new states would come to each others' aid in defense, for example, without giving any centralized authority for the raising of troops or the payment of costs necessary to maintain militia. Each state recruited and paid for its own forces, which soon led to troops not being equipped or paid.
The Constitution established a strong central government which was created to be superior to that of the state governments. The Constitution presents a long list of specific functions which were assigned to the Congress of the United States instead of left to individual states. The Constitution also set forth the "elastic clause" allowing the national government to expand its powers as needed to react to changing situations in the future.
While all powers not specifically delegated to the national government were reserved to the states, the Constitution created an organization for united action and government that allowed the United States to bring together the diverse interests of the different states into one country.