I finished David Novak's Natural Law in Judaism. On page 163, Novak quotes Sifre Devarim, no. 343, p. 397, which states:
"Not only did [the Gentiles] not accept it [the Torah], but even the seven commandments which the Noahides accepted upon themselves they were unable to persevere in them, finally casting them off altogether. When the Holy-One-blessed-be-He saw this he therefore gave them to Israel...so Israel accepted the Torah with all its ramifications and details, plus those seven commandments in which the Noahides were unable to persevere and which they cast off. Israel came and accepted them."
According to this passage, the Gentiles struggled with the seven Noahide commandments, and so God did not burden them with even more commandments, the ones in the Torah. Novak says, however, that the Israelites (in a rabbinic tradition) were Noahides in the sense that they did observe the seven Noahide commandments, and so God entered into covenant with them. The Noahide laws appear to be like a schoolmaster that prepares Israel for the Torah, perhaps because they are a baseline of ethics.