The constitutional convention includes the amendmend in the constitution or bring a new constitution into effect. For this reason the constitution allows that two-third of the total states must propose a national constitution convention and propose what are amendmends which are required and important. After a long debate held, the right of vote is given to every state for bringing this constitutional amendmend or new constitution into effect. So to approve the constitution to bring into effect the three-forth of the states have to approve.
There was a number of reasons. The articles of confederation didn't enable the central government to collect taxes. There was no National court to solve international conflicts. And many states took advantage of a missing strong leading figure in the form of a president. The Articles of Confederation was week due to giving too much power to the central government.
