Miller's work was initially designed for the stage. In this, it cannot be a novel. Miller meant for the work to be produced on the stage and wrote it as a stage drama. The work could not be seen as a novel for this reason. Miller uses the stage as a way to present the fullest way in which Willy and his own beliefs can be shown, without the bias of a narrator and showing us what it means to be Willy. This would answer the first half of the question. I think that the work serves as a tragedy because it highlights Willy's plight. I think that the work is a tragedy because of how Miller configures Willy, as someone who is powerless to change all of the elements that conspire against him from an economic point of view. As Willy absorbs the definition of the American Dream which identifies success in terms of money, Willy becomes a tragic figure seeking to find happiness and success in a setting where it is impossible to hold. In this, Willy's predicament is a tragic one, yielding so much in terms of painful revelation and emotional distancing from others. I think that this is where the drama can be seen as a tragedy.
Miller's work is meant to be acted on stage, and can't be said to be a novel and to call it a tragedy is reasonable as the main character dies with his dreams. W illy loman is a very intresting vision example and a victim of the american success myth as he still hold on to his dreams when he dies. Willy loman died an unsuccessful man without realizing his dreams, he died in the struggle. It's a tragedy