Pesach is the foundational holiday of the Jewish people, and the Exodus is the defining moment of Jewish history. A group of slaves formed a nation that, a mere seven weeks, later stood at Sinai so they could become a nation of priests and a holy nation. Mitzvah after mitzvah is observed zecher lyetziat mitzraim, to remember the Exodus. And these mitzvoth are in addition to the stand-alone mitzvah to remember the Exodus.
comparison of Purim and Pesach
“And it was at the end of four hundred and thirty years, in the middle of that very day, that the legions of G-d went out from the land of Egypt” (Shemot 12:41). After 430 years, does it really matter what day and what time of the day we left Egypt?