Usually we introduce all mitzvot bein adam lamakom (commandments relating to Hashem) with a birchas haMitzva (blessing). The mitzva of sippur y'tziat mitzrayim (retelling the story of the Exodus) as fulfilled through the recital and exposition of the haggada seems to be a notable exception. Both the Rishonim and Acharonim offer a variety of fascinating solutions to this question each one with ramifications beyond the haggada itself. Among the answers: there is no b'racha on a mitzva without a defined limit (Rashba); no blessing is recited on a mitzva whose primary kiyum (fulfillment) is in the mind or heart (Maharal); the haggada itself is structured in a b'racha format ending with "boruch ata Hashem, ga'al Yisrael" and we do not recite a b'racha on a b'racha (Ma'asei Nissim, Chida).
Perhaps we can suggest another answer based on concepts developed by Avudraham and Rav Y. D. Soloveitchik zt"l. Avudraham questions why there is no blessing on the k'riat sh'ma. He answers that the purpose of a b'racha is to be m'kabeil 'ol malchus shamayim (acceptance of the rule of Heaven) upon us before we perform the mitzva. We perform Divine commandments as instantiations of accepting Hashem's sovereignty over the entire cosmos, of the world, of our nation, and our individual selves. K'lal Yisrael first had to accept the whole package of Torah through the declaration of "na'ase v'nishma" before actually dedicating their lives to Divine Service. In the language of Beis HaLeivi (Mishpatim), only after we offered ourselves as servants of HaKadosh Baruch Hu did we become bound in the totality of Torah for all generations. The prospective convert to Judaism must reenact the same process by first accepting the totality of Torah and mitzvos; only then can his conversion be valid and meaningful. According to Avudraham, we reaffirm this broader commitment each time we perform a mitzva. Hence, before reading k'rias sh'ma whose very essence expresses a commitment to the dual themes of kabbalas 'ol Malchus Shamayim and kabbalas 'ol mitzvos (acceptance of the yoke of commandments), no blessing is necessary. We need not accept Divine sovereignty in order to accept Divine sovereignty.
The same approach can be applied to the haggada. The third section of k'rias sh'ma contains z'chiras y'tzias mitzrayim (remembrance of the Exodus). Rav Soloveitchik (see Haggadas Si'ach HaGrid) noted that Rambam (Hilchos K'rias Sh'ma) seems to include all three sections of the sh'ma in the Biblical commandment of reading the sh'ma. He explained that the Exodus is integrally linked to the theme of kabbalas 'ol Malchus. Through the entire experience of y'tzias Mitzrayim -- beginning with Moshe's demand of Pharaoh in the name of Hashem to release His nation, moving through the cataclysmic overturning of the rules of nature by means of the ten makkos, and culminating with k'ri'as yam suf with the attendant Divine Revelation enabling the Jewish people to prophetically utter the "Az Yashir" in unison -- Hashem demonstrated His Existence, His creation of the world as evidenced by demonstrating His utter mastery over nature which He created and manages, His Omniscience, and His Providence (see Ramban, end of Bo). These majestic events serve as the foundation of our acceptance of Hashem's rule over the world and our obligation of loyalty toward Him. Hence, remembering the Exodus brings to the fore the source of our acceptance of Divine rule and therefore His mitzvot.
In light of this, no blessing need be recited before the retelling of the Exodus for through its detailed and elaborate recounting on the night of the Seider we are reaffirming our loyalty to Hashem Yisborach. Therefore, there is no need to precede the haggada with the same theme through a blessing. Through our reacceptance of Divine sovereignty in the darkness of the world of hesteir (concealment), may we merit the open Revelation of Hashem's Presence in the Third Beis HaMikdash -- v'nochal sham min ha'Z'vachim umin ha'Psachim!