The Maharatz Chajes answers why, in the Haggadah, we do not immediately reply to the Four Questions with Rabban Gamaliel's statement that "Whoever does not say these three things..." in which the questions are answered. From The Students' Guide through the Talmud, pp. 198-200:
With regard to the questions found in Num. R. and Deut. R., and in Midrash Tanhuma and the Pesikta Rabathi, with which these books often begin their homilies, namely the halachic dissertations which are introduced with the words 'There is a ruling that an Israelite, etc.,' or with the words 'May our teacher instruct us', and followed with Aggada, we cannot trace the use of a similar formula in the Talmudim (Babylonian or Palestinian), except once in the Babylonian Talmud... [The halachic question is asked] and the Aggadist goes on to indulge in sophistry, changing one subject for another until he closes his homily with the words 'and as for the question which I have been asked...' and answers it...
The chief object, however, of the Babylonian and Palestinian Talmudim was to enlighten us either on halachic or legal decisions or upon matters of aggadic or ethical character. For this reason the redactors of the Talmud (except in once case) did not embody such a form of teaching in the Gemara, but dealt separately with the homiletical expositions in relation to the particular purpose for which they were needed, and separately also with the halachoth...
But it was not so with the Midrashim, which were not recognized as being for the teaching of halachoth, or for the publication of legal rulings needed for actual practice. In these Midrashim, the Rabbis at times left us their homilies either introduced with the words: 'There is a ruling that an Israelite, etc.,' or with the words: 'May our Teacher instruct us,' as indeed homilies were introduced when delivered to the masses in those days.
One may also note that the Passover Haggadah,[1] the compilation of which is thought to have taken place during the time of the Temple, as recorded in the Mishnah, Pes. 10, 4,[2] also introduces subjects with questions in the same way as the Midrashim of the earlier days. At first the children are made to ask four questions about the difference between this (the Passover) and other nights, and then the celebrant begins to narrate the successioin of events and to expound the Exodus and various passages in the Scriptures relating to this subject, and finally he turns back to reply to the question which had been put to him. So, for example, in reply to the question: 'Why are we eating this matzah?' he says: 'Because the dough of our forefathers did not have time to be leavened.' And there are similar replies regarding the Pesach (sacrificial lamb) and the maror (bitter herb). In this way the familiar difficulty which is raised as to why the narrator does not proceed at once with his proper answers to the questions on the reasons for the eating of matzah and maror is met, if we bear in mind the above elucidation of the methods adopted by the Tannaim and Amoraim in their homiletical discourse, as we have seen them exemplified in connection with the question referred to above (Shab. 30a), viz. that their way was to pass from one subject to another related to it, until they came back finally to reply to the main question asked.
[1] The ritual recitation for the Passover Home Service. Its name is derived from the word והגדת 'and thou shalt tell'. Ex. XIII, 8, and it includes the narration of the Exodus.
[2] Where first mention is made of the ritual and where R. Gamaliel is reported (Mishnah 5) as saying that 'one who has not said (i.e. not understood the spiritual implications of) these three words, Pesah, Matzah, and Maror has not done his duty'. The opinion is held by many scholars (see J.E. VI, 141) that this R. Gamaliel was the first of that name (who lived during the Temple) because he speaks of the Passover lamb. But even according to the view held (Weiss, Dor, II, 74) that he was R. Gamaliel II the mere fact that R. Gamaliel II speaks of a familiar ritual proves that the Haggadah was already in existence before his time. The proof however which the author has probably also considered was R. Tarfon's statement (Mishnah 6) in connection with the order of the Haggadah. R. Tarfon had lived during the Temple time (See Jer. Yoma III, 7). See also glosses on Nid. 6b by the author.