“The prevailing ideas entertained by him [Jefferson] and most of the leading statesmen at the time of the formation of the old Constitution were, that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally and politically. It was an evil they knew not well how to deal with; but the general opinion of the men of that day was, that, somehow or other, in the order of Providence, the institution would be evanescent and pass away. [...] Those ideas, however, were fundamentally wrong. They rested upon the assumption of the equality of races. This was an error. It was a sandy foundation, and the idea of a Government built upon it—when the "storm came and the wind blew, it fell."” —Alexander Stephen’s, VP of the Confederacy https://en.wikipedia.org/wiki/Cornerstone_Speech
https://www.monticello.org/thomas-jefferson/jefferson-slaver...
Let's face it, most of the founding fathers were racist slave owners.
Slavery was introduced in the New World and not in Europe, because that's where the non-Europeans were. When the native Americans proved poor slaves (among other things, dying in droves from disease and being better able to escape due to knowing the area), they imported slaves from Africa.
The Africans had been practicing slavery all along, but the Europeans had considered it barbaric until then. They still didn't tolerate it within Europe, but only safely out of sight in the New World. The institution of slavery only came into existence after colonizing places inhabited by people of other races.
Slavery (as seen then) implies that the person you are enslaving as subhuman (remember the 3/5s compromise?). Most slave owners treated their slaves as investments like a farmer would treat a tractor today. In fact, for the really terrible work many slave owners would hire Irish immigrants to do the jobs they didn't want their slaves doing out of fear of the risk of harm to the person doing the job. They were tools, not humans.
Racism, which we have now (though probably to a much lesser degree than what some people would want you to believe) acknowledges that the other is human but is inferior due to skin color or ethnicity.
Bigotry is more cultural. The other is human, and should have all the rights thereof, but the bigot doesn't like the culture of the other person. This is called racism by contemporary standards, which is very wrong.
The opposition was to count them for 0. The "compromise" was to account them for "only" 3/5 -- not at "3/5 of a human" but "slave owners get that much thumb on the scale of all future votes".
That still wasn't sufficient to mollify them, and they ended up seceding, making it quite clear that their reason was to continue to own people.