Congress called for the Constitutional Convention because the Articles of Confederation, America's first constitution, was highly ineffective. The Articles of Confederation had several different weaknesses including the federal government's inability to collect taxes, states were allowed to ignore federal laws, and the inability of the federal government to create a standing army.
Originally, Congress wanted to merely change the Articles of Confederation. However, after realizing this would be an extremely difficult/near impossible task, they decided to create a brand new constitution.
