The Constitutional Convention is one of the important parts of the US history because it creates our new constitution and gives us a new government. It took place to check and solve the problems with the Articles of Confederation. But Madison and Hamilton wanted to create a new government rather than solving the old problems. There are a lot of reasons that the convention took place.
One of the main reasons for the convention is the weak central government which was existed under the Articles of Confederation; the government faced a lot of difficulties that created the opportunities for Madison to draw the sketch of the Virginia plan. Madison came out with every single possibility to make this plan successful.
There is another reason for the convention which is slavery. Slavery was not forbidden in that time and most of the delegates owned slaves. The new constitution could regulate the slavery only by limiting the international import or export.
There is another reason for this convention which is the problems with the Articles of Confederation. It does not protect the nation against any foreign attacks and does not secure the harmony of the states. It could not protect itself from its own attacks. So, the new constitution became necessary and it took place.