The Articles of Confederation were written in fear of turning into another Great Britain, so they made the Articles really loose, and they couldn't achieve anything. For example, 13 out of 13 states...
We had the power to tax and borrow money so the govenment could actually do stuff. Also originally we only had a legisltative branch to make laws and that was it. The gov had no way to enforce laws....
Because the Articles of Confederation had no president and called for a weak national government. The new constitution gave more power to the government and separated the government into 3 branches.