User blog comment:Yellow Ledbetter/New Assassin?/@comment-4357842-20120302043858/@comment-1153722-20120302095502

Desmond can only relive the memories that are in his DNA. Genetic memories are passed on via conception; when somebody has a child, the memories of that person up to the point of exception are passed on to the child, nothing more. Therefore, we can only view the memories of this ancestor up until the point that he has the child who is Desmond's ancestor.

This is also the reason why we didn't see Altaïr's death when reliving Altaïr's memories but only through Ezio using the Memory Seals. And we only saw Ezio's death through Embers, which was not a part of Desmond reliving Ezio's memories but a standalone close to Ezio's life.