Answer: Because the Articles of Confederation was terrible.
Explanation: The AoC was terrible because it later lead to states "leaving" the U.S. and forming their own "countries". This lead to a massive problem because almost every state had a different currency which lead to trade problems. There are many other examples but this is just one of them. Even though some people wanted to keep the AoC but just alter it,the vote happened and a new Constitution was made, with is called the U.S. Constitution.