Why did the founders develop federalism?

A federal system of government is one in which power is distributed between a central authority and smaller political units, such as states. Advocates of federalism claim that this method of governance is to be preferred as it brings power closer to the people at the local level. In countries with large, diverse populations, federalism can be a better way of giving people a voice in how things are run. A federal system can also act as a check upon the central government overreaching itself and expanding its powers beyond what the voters think acceptable.
The framers of the Constitution designed a federal system because they thought it would prevent central government from having too much power. The American colonists had just fought a long, bloody war against what they perceived to be the tyranny of the British. The United States had been ruled from a centralized government far away in London, and the result had been a systematic assault on American liberty. So the framers set about constructing a system that would protect Americans' hard-won freedoms—a system in which a considerable degree of sovereignty would still reside with the individual states.
However, the question of the precise division of power between the federal government and the states was a bone of contention during the Constitutional Convention. Some argued that the proposed new Constitution actually gave too much power to the federal government, whatever the framers' intentions, potentially turning it into an instrument of tyranny greater than anything devised by the British. Others argued that leaving ultimate sovereignty in the hands of the states, as had been the case under the Articles of Confederation, would leave the United States unable to speak with one voice on matters of concern, such as foreign affairs and the economy.
Given such radical differences of opinion, the federalist system that subsequently emerged from the Constitutional Convention was, inevitably, something of a compromise. But the precise terms of the distribution of power between the federal government and the states would remain highly contentious for many years to come, leading indirectly to the Civil War.