The Founding Fathers chose to adopt the Constitution primarily to outline the powers of a new federal government. After experiencing the weaknesses of the Articles of Confederation, which did not provide a strong enough central government to address various challenges (such as economic instability, interstate conflicts, and lack of a unified national defense), the Founding Fathers convened to create a new framework for governance.
The Constitution was designed to establish a more effective federal structure that would balance power between the national and state governments, define the roles and limits of the government, and provide a system of checks and balances. While it also included mechanisms to limit the powers of government (such as the Bill of Rights), the primary motivation was to create a solid foundation for a functioning government capable of meeting the needs of the country.