Why did God add rituals and regulations to the Ten Commandments?
The rituals and regulations were added so God could bring in the religious system of the Law. That system included civil rules for dealing with one another, priestly stipulations, and tabernacle instructions. The Law itself was given to Israel, not the Gentiles, although it covered the whole human race.
Les explains that in order to bring in the religious system of the Law, God had to add to the Ten Commandments all the rituals, regulations, rules, and commandments for dealing with each other—the civil law—and all the stipulations for the priesthood, which is the ecclesiastical part of the Law. God also gave Moses instructions for building the tabernacle, and Israel accomplished that while in the wilderness of Sinai. This giving of the Law was all part of Israel's heritage. The Law was never given to the Gentiles; it was given to the nation of Israel, and no one else, though as Romans chapter 3 shows, it covered the whole human race.