The plantation system became dependent upon slave labor during the 1700s because agriculture became more popular in the Southern colonies and they ended up heavily relying on plantations for their source of income.
During this time, the South was primarily getting their income from plantations and growing cash crops. Cash crops were crops that sold for a lot, mostly overseas. Southern colonists would focus on selling cash crops and trying to expand their plantations, which caused them to need a lot of slave labor. First, they focused on mostly indentured servants, people who were trying to repay their masters for buying their passage to the colonies, but when that deemed to be too much work, many Southerners turned to slaves for their labor.
The North was not focused on agriculture like the South was, they were focused on industrialization. They did not need slave labor in order to maintain their income, so they did not have to worry about if what they were doing was morally wrong or not.
