The capitalist class doesn't really care so much about the specifics of the labor relations they are engaged in, as long as there is a reserve army of labor and exploitation. Having slaves makes it all the easier to extract surplus value. The problem the capitalists had with serfdom specifically was that it tied peasants to their land, which hindered their movement across industries. The problem they had with antebellum slavery was that the plantation owners did not care to mechanize agriculture, leading to capitalists needing to pay higher wages to workers in order to keep them alive (and also limiting the number of people who could be moved out of agriculture and converted into industrial workers).

On a somewhat related note, the issue of settler colonialism cannot be overlooked. Many of the abolitionists wanted to get rid of slavery because they believed (correctly) that it created a politically dangerous subclass (the slaves) who could topple the whole imperial project of America, and undermine the racial purity of the country.