Why did they get rid of the articles of confederation?

The basic answer is that the Articles of Confederation just didn't work. We were not the "United" States. The federal government had almost no power because of the strong anti-federalist sentiments of the time. People were afraid of a strong central government and wrote the AOC to give power to the states. The federal government couldn't even regulate money. The AOC didn't give enough power to the federal government that would allow it to successfully work. 

The need for a stronger Federal government soon became apparent and eventually led to the Constitutional Convention in 1787.