The necessity for a precisely drafted 'national' governing rules, is the right answer.
The colonies of America created the individual constitution for their states. The Continental Congress served as the common legislative party for all these states. but in the year 1777, the members of the Congress party realized the necessity for a precisely drafted 'national' governing rules and thus drafted the Articles of Confederation. Therefore, the Articles of Confederation served as the first constitution of the United States and created a national government with a very limited power given to it