Why was the situation with slavery in the United States more complex than in other areas?

Not long after Columbus set sail for the New World, the French and Spanish brought slaves with them on various expeditions. Slaves accompanied Ponce de Leon to Florida in 1513, for instance. But a far greater proportion of slaves arrived in chains in crowded, sweltering cargo holds. The first dark-skinned slaves in what was to become British North America arrived in Virginia — perhaps stopping first in Spanish lands — in 1619 aboard a Dutch vessel. From 1500 to 1900, approximately 12 million Africans were forced from their homes to go westward, with about 10 million of them completing the journey. Yet very few ended up in the British colonies and young American republic. By 1808, when the trans-Atlantic slave trade to the U.S. officially ended, only about 6 percent of African slaves landing in the New World had come to North America.

To the point slavery was so bad because rebellion's triggered more rebels  and they were fighting back for freedom some of them and other continued to work as slaves causing a little confusion in the slavery systems.  

c, because the american revolution was from 1765 to 1783.

c. free trade can lead to low pay for workers in poorer countries.