Sinai left no room for free will — Purim, centuries later, was designed to require it.
Q. Why Don’t We See Supernatural Miracles Anymore?
The Six Day War and the present Ingathering of the Exiles are natural miracles. Supernatural miracles were necessary for the nascent nation of Israel, who grew up as idolaters in Egypt, and needed a strong and conclusive “introduction” to G-d. Just as the individual matures, so too, with time, the nation and mankind mature, as well. The Talmud (Shabbat 88a) explains that Israel did not really have free will at Mount Sinai, because all of the supernatural miracles were so convincing. By the time Purim came around about 800 years later, G-d chose to save us through natural actions, which leaves more room for free will. In the megilla, the name of G-d purposely doesn’t appear, so that man can decide freely to recognize and thank Him, or not. The Talmud continues and explains that’s why Purim was an even higher level of accepting G-d than at Mt. Sinai. This is a more mature and deeper way of choosing to accept Hashem through identification with His running of history and ideals and not by force, which also adds to our enthusiasm of observing the mitzvot, which we accept voluntarily.
With Love of Israel!
Rav Ari Shvat