The Articles of Confederation could not be fixed (in the minds of the men who gathered at the Constitutional Convention in 1787) because they simply did not have a strong enough federal government and, relatedly, because they made the 13 states practically independent countries. To fix these problems, the delegates would have had to amend the Articles so drastically that it was easier to write a new constitution. Moreover, all 13 states would have had to agree to amend the articles while the Constitution stipulated that only 9 states had to ratify it for it to go into effect.
The delegates to the convention believed that the United States needed weaker states that were not practically independent. The country needed a stronger national government that could have power over the various states. The Framers felt that the Articles created a situation where the United States was too weak. The national government could not impose taxes to pay for an army and navy. It could not prevent the states from engaging in trade wars against one another. In short, the country was not very united and was not very strong.
Because the problems were so drastic, they decided a new constitution was needed. This new constitution would need to remake the relationship between the states and the federal government. It would need to give the federal government more powers. It would need to actually have an executive branch with a strong leader. These were changes that were too fundamental to be put in the form of amendments. Moreover, the Framers realized that not all of the states would necessarily agree to the amendments, which would have ruined their plans for a new system. Therefore, they jettisoned the entire Articles of Confederation and wrote a new constitution.