When we approach the holiday of Pesach, we begin by separating the various Halachot of the Chag. We must first look at which laws are from the Torah.
On Seder night, there are two Torah laws. The first is the obligation to eat Matza. We are to eat a כזית of Matza which equals one slice on three occasions. The first is for the blessing over the Matza. The second כזית is when we make the Hillel "sandwich". And the third כזית is when we complete our meal with the eating of the Afikomen.
The second Torah law at the Seder, is to tell the Pesach story. This is learned from the words והגדת לבנך, "and you shall tell your children." We fulfill this Mitzva by focusing our Seder on the children. We devise all kinds of tricks to keep the children awake and involved throughout the night.
The climax of telling the story is when we quote Rabban Gamliel who said that minimally we must explain פסח מצה מרור. That is, we are to speak of the Pesach sacrifice, as well as the reason for eating Maror and Matza.
So we begin our Seder with fulfilling carefully the Torah laws and we then move on to the rabbinic laws.