In Sukot, the Israelites baked the dough that they had brought out of their homes in Egypt into cakes of matzah, since it had not leavened, for they had been driven out of Egypt and could not delay, nor had they prepared any other provisions for themselves.
Exodus 12:39
Even though they were traveling into the desert, they relied on G-d to provide for them. G-d accounted this to their enduring merit. Miraculously, the bread they now baked sufficed for 61 meals, and was as tasty as the manna they would eventually receive from G-d.
Even though the journey from Raamses to Sukot took a miraculously short time, when we consider the time it took for them to set up camp, kindle fires, and start baking the dough, there was more than enough time for the dough to start to rise. Nevertheless, it did not rise.
This was because the spiritual dimension of what was happening influenced the physical reality. Spirituality, the Jews "had been driven out of Egypt" -- they were uplifted to such spiritual heights that they totally transcended all semblance of ego and their consciousness was filled with G-d's absolute, all-encompassing reality. In this context, the dough could not rise, for leavening reflects the haughtiness of ego. Leavening causes the dough to rise and puff up into an aggrandized version of itself, just as the ego aggrandizes our sense of self far beyond what is real. This matzah did not rise even though there was no specific command to prevent it from rising -- or even to eat matzah altogether.
The cause of this spiritual ascent was the revelation of G-d's infinite transcendence that occurred at midnight of the 15th of Nissan. The matzah the people ate on the morning of the 15th of Nissan was part of this post-revelation reality, so it simply could not rise, and there was no need to take any precautions to prevent it from rising. In contrast, the matzah that the people were commanded to eat in the future when they would each year celebrate the festival of Passover had to be protected from leavening.
The matzah we eat each year at the Seder, however, must be eaten before midnight, recalling the matzot we ate on the night of the Exodus before midnight. Inasmuch as this matzah was part of the pre-midnight reality, it must be carefully prevented from rising.
From Kehot's
Chumash Shemot