Final answer:
The primary factor in the spread of slavery in North America was the high demand for labor to cultivate and harvest profitable cash crops such as tobacco, rice, and indigo, necessitating a large and permanent labor force.
Step-by-step explanation:
The spread of slavery in the colonization of North America was primarily due to the demand for labor because of cash crops like indigo, tobacco, and rice. These crops were highly profitable and necessitated a large labor force for their cultivation and harvest. Initially, indentured servants were used, but their limited service terms and insufficient numbers led to the increased importation of enslaved Africans. This labor source proved to be more permanent and plentiful.
Slavery became deeply entrenched particularly in the Southern colonies where agriculture was the backbone of the economy. The institution of slavery created an economic disparity between the North and South, which would eventually contribute to the ideological and geographical divisions leading to the Civil War.