American leaders sought to draft the Constitution as a new plan of government. What had the previous plan failed?

The Articles of Confederation.

The Articles of Confederation failed for one simple reason; weakness. The first "constitution" failed was due to the following reasons:

It did not create a sufficiently powerful centralized government Congress was designed poorly with only one house, and one vote per state High state majority to pass laws (At the time, there were only 13 states, and the AOC said that 9 or those states needed to vote for something to pass it, which is a 69% "supermajority"). Levying taxes was difficult for the government States had their own unique systems (money, foreign policy, etc)

Eventually, all these reasons created a very difficult and poorly designed government. Shay's Rebellion in 1786-1787 ultimately marked the end of the Articles Of Confederation, as it showed how weak the central government truly was. The U. S. could not protect a perpetual union, and Founders of the AOC pushed to create an entirely new and more effective document to lay the law of the land. And so, the U. S. Constitution was born in 1787, and it continues to be a flexible document to this day.