Why did the delegates change the constitution?
I believe the question you are asking is why the delegates at the constitutional convention changed the Articles of Confederation and replaced it with Constitution. It is on this premise that I will base my answer.
By the mid-1780s, it was clear the Articles of Confederation, which was our first plan of government, had significant weaknesses. Designed to be a plan of government that limited the power of the federal government, it was clear changes had to be made. We had significant financial problems. Because both the state government and the federal government issued money, inflation became an issue. People were unwilling to accept paper money, fearing it had little value. Because Congress couldn’t tax, it was difficult to pay our debts to other countries.
Our government was having difficulty maintaining order at home and abroad. When farmers in western Massachusetts rebelled because they were losing their farms, the federal government stood idly by as the state militia dealt with the crisis. When Britain and Spain interfered with our trade or encouraged Native Americans to attack us, there was little we could do because our government couldn’t require people to join the army. Plus, there wasn’t enough money to pay for and supply an army.
These issues and others led to the call for a meeting to discuss the possibility of writing a new plan of government. At the constitutional convention, a new plan of government was written and eventually adopted by the states.