I would suggest a few reasons. If Israel was to simply follow the Divine, doing what they were told, that would fundamentally be no different than being slaves to a master. They need to learn that Judaism is a religion of both pure faith and unquestioning observance. What we do in our daily lives, and the way in which do it, connects us inexorably to Heaven. The reverse is true, as well; G-d is our inspiration to stay on the right path, both ethically and ritually.
To take it to the next level, however, Israel needs to learn that the Mitzvot they are being taught are not random tasks of hard labor which they are being compelled to perform. The laws of keeping Hebrew “slaves” is not a construction which legitimizes slavery. They focus on the moral imperatives of keeping servants and treating them fairly, including freeing them after 6 years of service. The Rabbis offer numerous situations in which offering someone the opportunity for indentured servitude would be a positive act, such as rescuing them from poverty or the ability to pay off a debt. The laws of property, agriculture, and livestock are Mitzvot which teach the nation that there is a need for communal responsibility once living in a society where, for the first time, they will have their own property and domiciles. These laws are based on equity and fairness, not pointless obedience.
And then, as if to restate His point, G-d returns to the concept of holiness toward the end of the Parsha, reminding the nation to eschew idol worship, promising them deliverance from the Canaanite nations, and prosperity in the land. Israel gets the message. They say together, as one, “All the words that G-d has spoken, we will do, and we will obey.” Then, and only then, do Moshe, Aaron and his sons, and the 70 elders of Israel have a mystical experience where G-d reveals himself (in some form; the Rabbis disagree) in a manner far more visual than that which even Moshe saw before or hence.
Our people have started to learn. From now on, the balance of faith and action is established.Rabbi/Hazzan David B. Sislen