It was slavery. Just look at the documents of secession.

As an example, the opening paragraph of Georgia's declaration of secession: "The people of Georgia having dissolved their political connection with the Government of the United States of America, present to their confederates and the world the causes which have led to the separation. For the last ten years we have had numerous and serious causes of complaint against our non-slave-holding confederate States with reference to the subject of African slavery."

The second paragraph of Mississippi's declaration: "Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization. That blow has been long aimed at the institution, and was at the point of reaching its consummation. There was no choice left us but submission to the mandates of abolition, or a dissolution of the Union, whose principles had been subverted to work out our ruin."

Any claim that it was not about slavery is revisionist history at best.

This is a Frequently Asked Question, and appropriately, there are several great answers you should read in the FAQ section of this reddit.

Cheers!

The right to have slaves.