When I was a child, my father the Rebbe Rashab asked me why we don't make a Bracha on the Mitzvah of telling the story of Yetzias Mitzraim on the Seder night. I did not know.
He told me that he was asked the same question by his father, the Rebbe Maharash and that he too did not know. The Rebbe Maharash then told his son, that he was asked the same question by his father the Tzemach Tzedek and that he too did not know. The Tzemach Tzedek told him that he had been asked the same question by his grandfather the Alter Rebbe and he too had been unable to answer. But his uncle, the Mitteler Rebbe, who was present at that time, answered with the opinion of the Ri”f. The Ri”f holds that since once has already been Yotzai with the Bracha of Kiddush in which Yetzias Mitzraim was mentioned.
There is also the answer of the Rashba that since you are Yotzai the Mitzvah with the smallest remembrance of Yetzias Mitzraim mentioning, we apply the rule that there is no Bracha for a Mitzvah that has no measurable amount.
The Alter Rebbe questioned this explanation, because on the night of Pesach there must be much telling of the story and a mere mention is not enough. The Alter Rebbe answered that it is like Birkas Hamazon. Even though it is a Mitzvah, we do not make a Bracha that Hashem commanded us to say the Birkas Hamazon. This is because Birkas Hamazon itself is a Bracha. So too, we don't make a Bacha on the reciting of the Haggadah, because the whole Haggadah is a blessing (praise) of Hashem.
~ First night Seder 5697
