In Death of a Salesman, why do so many readers consider Biff to be the true protagonist, rather than Willy?

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.