Colonial slavery oppressed conquered people's on their local soil where they were the majority. This is hard to keep up for a global power constantly at war with its neighbors.

American slavery oppressed ethnic minorities ripped from their homeland and taken to a different continent. Slaves had a harder time coordinating rebellions as they were not a single conquered peoples, and were in the minority so couldn't count on winning battles of attrition.

American slavery, like the Roman version it was patterned against, could have continued for hundreds of years if not for the Civil War.

Colonial slavery though was a lost battle from the start. Colonial powers needed to develop their colonies, but the moment you do that you increase the chance of successful rebelion. The only way to defuse it is to reduce the oppression.

The Founding Fathers didn't have a shared opinion on the issue, and the only concrete action they jointly took was directly protect it and to bake in extra political power for slavers.

> but had to compromise lest the fragile Union break before it could form

The people those founding fathers who wanted slavery eventually eliminated had to compromise with were also founding fathers.