The decline of slavery in the Roman Empire can be attributed to several factors.

Economic Changes:
- As the empire expanded, there was an increase in the availability of cheap labor from conquered territories, leading to a decrease in the demand for slave labor.
- The development of new technologies and agricultural practices also reduced the need for large numbers of slaves to work in fields and mines.

Social and Political Factors:
- There was a growing opposition to the institution of slavery among the Roman elite, who began to see it as morally reprehensible.
- The spread of Christianity, which emphasized the equality of all human beings in the eyes of God, also contributed to a shift in attitudes towards slavery.

Legal Reforms:
- Emperors like Augustus and Hadrian introduced laws that granted slaves certain rights and protections, making it more difficult for slave owners to mistreat their slaves.
- The gradual erosion of the Roman legal system and the breakdown of centralized authority also made it harder for slave owners to maintain control over their slaves.
Overall, a combination of economic, social, and political factors led to the decline of slavery in the Roman Empire, ultimately contributing to the fall of the empire itself.