We're not thinking sufficiently outside the box here.
What socio-economic conditions and political events could have created a situation that when American colonies fought for independence against Britain, banning slavery wouldn't be a non-starter?
I really do not see banning slavery as a non-starter. It would take more than one change, but with the right butterflies slavery could be abolished at the Constitutional Convention. It would happen over a period of time, probably about 20 years, but it could be done. I do not see the slave owners as all powerful even in OTL. If they were there would have been no 3/5th compromise and slavery would not have been mentioned at all in the Constitution. Please see my previous comments on some ideas to diminish the power of the plantation owners.
Not all powerful, but powerful enough to insist on the three-fifths rule - the North didn't want slaves to count at all for representation.
Push the Southern Staes any further and they just don't ratify. Even OTL, Virginia ratified only by 89-79, and NC waited a year before ratifying at all. Anything provocative just kills the Constitution.
You're just replacing one problem with a larger problem.What if the British Empire stopped transporting slaves long before, so there were far fewer slaves? Would the southern colonies have existed without extensive slave labor?