Slavery continued to exist in Western Europe after the fall of Roman empire until at least year 1000 AD.
In other former parts of Roman Empire such as Northern Africa slavery continued to flourish almost until the 19th century.
On what basis are you making this assumption? "The fall of Rome" was not an anti-slavery uprising. It was gradual disappearance of the central authority (and all its benefits such as roads, law and order etc).
About 10% of England's population entered in the Domesday Book (1086) were slaves. Compare that with the Roman Empire where slave population (including Rome and all provinces) are estimated at 10-15% of the total.