Final answer:
Federalism was adopted at the Philadelphia Convention to create a balance between a strong central government and states' autonomy, steering clear of the shortcomings of the confederacy and the oppressive nature of a unitary system.
Step-by-step explanation:
At the Philadelphia Convention of 1787, Federalism was chosen over a confederacy or a unitary system because the confederation established under the Articles of Confederation had proven too weak to address national concerns effectively. Economic challenges, discontent with foreign policy issues, and internal disputes among the states highlighted the inability of the confederacy to act cohesively on behalf of the new nation.
A unitary system was not favored as it resembled the centralized authority of British rule from which the American colonies had just gained independence. The delegates sought a balance between a strong centralized government and the retention of state autonomy to avoid the pitfalls of both a confederacy and a unitary government. The result was a new federal system that divided powers between a national government and state governments, seeking to ensure that neither would dominate the other, balancing national unity and regional interests.