It is generally forbidden to have a lengthy interruption between Kiddush and the eating of the meal. The rule is “Kiddush may not be made except in the place where the meal is eaten” (Pesachim 101a), and it applies to proximity in time as well as place. Why then is an exception made at the Seder, when the meal is not begun until an hour or two after Kiddush?
Rav Shlomo Zalman Auerbach zt”l explained that the Haggadah should not be seen as an interruption, but rather as an extension of the meal. In order to eat the Matza and Maror, a person must first reach a certain level of spirituality and mental preparedness regarding what he is about to do. “In every generation a person must consider himself as if he personally had gone out of Egypt” (Pesachim 116b). the purpose of reciting the Haggadah is to build up to the climax of eating the Pesach foods. Thus, far from being an interruption before the meal, the Haggadah is itself the preparation for, and the beginning of, the meal.
R’ Shlomo Zalman also pointed out several other indications of this fact. The Matza is broken almost immediately after Kiddush, long before the Beracha is to be recited over it. Why is this done? Would it not be more logical to break the Matza just before it was eaten (as is, the fact, the Rambam’s opinion)? The answer is that it is broken as we begin reciting the Haggadah to indicate that the Haggadah itself is, as we have explained, in reality a prelude to the eating of the Matza. Furthermore, R’ Shlomo Zalman concluded, our explanation also accounts for why על הגפן, the blessing after the wine, is not recited after Kiddush, as it is recited whenever one drinks wine and does not immediately eat a meal. Here too the meal actually begins immediately after Kiddush with the recitation of the Haggadah.