The roots of the consolidation of slavery lie in the staple crop system of agriculture in the South. During the early years of the republic, tobacco was the primary staple crop in many areas, along with rice and indigo in South Carolina. All were labor intensive and produced on a large scale. Slavery provided the necessary labor to sustain this system. It became entrenched and even expanded with the invention of the cotton gin, which allowed for more acreage to be devoted to cotton production. Cotton was very much in demand as a result of the textile revolution; and was also labor intensive. As a result, slavery became consolidated in the Southern states.
The Northern states did not rely on the large scale agricultural production that defined the economies of the Southern states. Such a development was impossible in the North as the climate, soil, and geography were unfavorable. The North therefore turned to industrial production as a major economic source. Industry did not readily lend itself to the use of slaves. Additionally, with the influx of immigrants in the early nineteenth century, a ready source of labor was available in the North. Immigrants could not move to the South, as there was no available land. Hence slavery offered no economic benefit to the North which soon focused on the brutality of the "peculiar institution."
The short answer must have been the cotton gin made cotton plantations profitable.
Both the North and South must have feared slave rebellions. A musem in New York City records one slave rebellion (I think in 1712). The museum has the remains and assoicated artifacts of slaves killed in the rebellion. The remains have teeth filed to points and tatooed skin, which the white population must have found frightening.
Combine that with the fact that cotton didn't bring a profit in the North, so the people in the North must have felt that slaves put them at too high a risk.