Bhishma got the boon of “ichcha mrityu” which means you can choose the time of your death. So when Bhishma was shot by arrows from Arjuna, the sun had just moved over the equator into the southern hemisphere. Bhishma was laying there on a be of arrows, shot so many times by Arjuna, he could lay down flat.
He said, “Well, I am not going to leave my body when the sun is in the southern hemisphere. I am going to wait until the sun goes into the northern hemisphere. So everybody gather around. Stop the war. All the disciples, all the grandchildren on both sides of the dispute, the whole family gather around me and I am going to tell you about the code of Manu and I am going to tell you about Indian philosophy and I am going to tell you about so many of the traditions and customs and heritage and I am going to tell you what you want to remember everytime of your death.”
And there were so many stotrams and stavs and many, many jewels that came from those discourses and they have been collected into many volumes. Much of it was included in the Mahabharat, some of it came in later traditions. Bhishma became the mouth through which all of the vedic knowledge was expounded and preserved. Whoever wanted to preserve a tradition or point of history said “Bhishma said” and it became a tradition just like Vyas.