I suspect being rich would make the people care Little about slavery. Italy during the Roman peek may have been twice as rich as average in terms of GDP and many of the Roman provinces may have been rich as well with the poorest provinces being maybe of average wealth in terms of World average GDP. So Rome during its peek was rich and had relative Peace which people maybe care more about than slavery whose conditions improved over time.
For many if not most people the Roman Empire was better than the alternatives even if it was terrible by modern standards, by ancient standards it would likely be what we consider to be a top tier nation to live in.
Alot of Romes wealth was based on trade if Im not wrong and far from everyone was a slave and not all slaves lived in terrible conditions some did have better living conditions than the average.