Book 02 - Exodus

Because slavery happened ... to God's Covenant people ... 'cuz they screwed up.
Exodus is also a foundational story of rescue from oppression, showcasing God hearing the cries of his people and acting to save them because of a promise gifted to Abraham.
The rituals established designed to keep an agrarian people focused upon serving a living God amidst state-aggrandized idol worship is the considerable justification for appealing to the earthly needs of what Moses himself terms as being a “stiff-necked people.”