Why is the play called "Death of a Salesman"?
On the literal level, it is, indeed, about the death of the salesman, Willy Loman. However, more importantly, it is a tragedy concerning the death of the American Dream that Willy Loman represents. Willy Loman realizes that as a salesman, he doesn't "put a bolt to a nut,he don't tell you the law or give you medicine" (Requiem); a salesman just sells, with nothing to show for his efforts, except, at the highest level, money, a materialistic reward that can be gone in a moment. Willy spends his life working for material goods, and at the end of the play, after his suicide to provide the family with money, his wife only has a house that is paid for. Linda says, "We're free and clear," yet her husband is dead. Thus, the American Dream, to have more things is actually the death of what once defined America, the strength of the American spirit. Willy Loman represents us all-those who become entrapped by the wrong dream and sacrifice life. This point is illustrated best by Willy's garden - his attempt to actually grow something, yet his house is surrounded by a "solid vault of apartment houses." Obviously, Miller uses strong imagery in the opening scene description illustrating Willy Loman's prison.
The title is ironic. A salesman does in fact die in the play, but the title points to the fact that this play is going to diverge from the tradition of great tragic heroes. Willy Loman is no hero, but an ordinary person who meets a tragic end.
In the past dramatic tragedies have usually ended with the death of the protagonist. Miller's play is no different in this regard, but it is who the protagonist is that is different. He is not a prince like Hamlet, a king like Lear, or a mythic god like Achilles. He's a salesman, and not a very good one at that. His death is not a major event, and his funeral is not even well-attended.
In my opinion...
It's because, the play is in fact about the death of a salesman, Miller doesnt at all define Willy's occupation, and clarify what he actually sells, so Willy is just a representation of every sales man, he's of insignificance: he himself and his whole life. Miller wants to just choose one aspect of the withering American Dream and present it through the death of this one salesman, it shows the dream is breaking down everywhere even in small jobs.
...Miller titled it Death of A Salesman to summarise the book on a literal level, it is then when you read it you see how the death of this one man has so much meaning, the title also adds to how insignificant Willy is, he's simply another American dream failure amongst the other millions.