Death of a Salesman, by Arthur Miller, certainly abides by its title for it definitely focuses on the character of salesman Willy Loman. Willy's immediate family and friends, and even his most beloved son, Biff, remain as secondary characters because their lives are a consequence of the actions and choices of Willy Loman. Therefore, no matter how closely Biff's life is analyzed in the play, his life is still an extension of Willy Loman, a consequence of Willy Loman, and a reaction to Willy Loman.
Another way to evidence this posture is to notice how, in the structure of the play, each of the Acts, including the final Requiem, are dedicated to the exploration of Willy's past, present and his final ending. We, as an audience, can appreciate the fact that we come full circle with Willy, and that we are "with him" until the end of his life.
Conversely, Biff's character is meant to represent every single one of Willie's hopes, dreams, and caprices. For this reason, we never really get to "know" the real Biff. We do not know his likes, except that he likes to work outside and dislikes being imposed upon. We know his weaknesses, and we know that he has been built up psychologically to think and act like Willy. We also know that his world comes crumbling down when he finds out about his father's infidelity, and that his life has been made of silly choices because he is, innately, lost. The life of Biff is not disclosed in its entirety, however, the life of Willy definitely is.
Finally, we do not even know what ends up happening with Linda, Happy, and Biff, other than learning what they plan on doing after Willie's passing. This is significant, because if Arthur Miller had wanted us to come to a full circle with those characters too, he would have most definitely given us that chance. Therefore, it is safe to conclude that it is Willy's life and circumstance what is given the center place.