A case can be made for either Willy or Biff as protagonist. However, it is DEATH OF A SALESMAN. Yet, the conflict between Biff and Willy is what drives the play. Noted playwright David Mamet said, and I'm paraphrasing, that in order for a scene to take place, two characters have to want something from each other. Willy wants success for Biff (and Happy as well, I suppose, although any dreams for Happy are never mentioned) and Biff wants Willy to tell the truth. Namely, he wants Willy to own up to the fact that all of the values he taught the boys growing up, the importance of being well-liked, personal attractiveness, having a likable personality, are all false values. As Biff puts it, Willy blew the boys "so full of hot air that [Biff} had to be boss big-shot in two weeks." The dramatic confrontation at the end of Act two puts both characters in center stage where they have it out. Perhaps people think it is "Biff's play" because he survives. He is the one who breaks the cycle of lies and impossible dreams. We know Biff will be ok and that Happy will more than likely grow up to be just like his father. If Miller had kept his orginal title of PRIVATE CONVERSATIONS, then Biff might be a more viable protagonist. He didn't, so what we have left is Willy's story, not Biff's.
One reason why Biff could be considered the true protagonist is because his character is more dynamic in terms of the current story line than Willy. What we know about Willy Loman we only witness through the memories of Biff and the delusional ramblings of Willy. Willy may have been dynamic in his past, but as of the present, Willy does not change at all throughout the current story.
Biff, however, does change eventually as a result of the events that took place once he faced his father and tried to call it a truce for the second time, without success. His character obtains an additional dimension that we cannot apply to Willy.
I, however, still think that the protagonist in this story is the salesman, Willy. It is his story and his own pre-conceived notions of success and the American Dream what landed his fate, and that of others, of course. But he was the one who came defeated amongst them all.
I believe that the reason would be because Biff has been raised to become what his father had expected of him, but never can live up to his father's dreams and expectations of him. He is not bullied into things but rather feels the pressure from his father.
For me, I don't identify him as the protagonist, because I see Willie as the one who is beaten down by life. However, if you put Biff in front of his relationship with Willie or even Willie's wife and Happy, they have all been victim to Willie's expectations and impression.