The answer is simple, everyone should know it. The answer is slavery. The US Government recognized that the US could not progress and modernize, nor freely trade with Europe without first preventing the expansion of slavery into the West, ultimately ending slavery, and beginning to guarantee some modicum of civil liberties to black Americans. Southern slave States found these notions so reprehensible they tried to form an entirely different Country, and immediately declared War on the United States. This is not complicated. Its not controversial. They proudly declared it. Black people were property, were inferior, and were meant to be slaves. More than one State's Article of Confederation even asserted that GOD deemed this so.
The level of either ignorance, or willful avoidance displayed by Nikki Haley here is comically absurd so egregious even Trump could avoid it. He'd have at least said some slick sh*t like "Yes the Civil War, very bad, very unfortunate. A lot of people say the it was caused by slavery, and some think it was State's rights! Everyone should have rights, rights are the oranges...the oranges...you know the beginning of all the freedoms!"
He or his handlers would have made sure no one could say he didnt mention slavery. Haley is just a racist or a coward, or both.