Willy just isn't good at selling anymore (if he ever was). Death of a Salesman is the story of a man who is pushed into fantasy by his failures in reality.
He can't provide for his family. He does not have the respect of his sons. The dreams he once had prove to be only dreams.
Willy's characteristic response to the facts of his life and the realization of his prospects is to cast blame on the world, as generally and specifically as he can. If believes that the world offers no more opportunities, it's because he feels that he has no power in the world to grasp opportunity. If he feels that things were once better, it's because he is unwilling to face the reality of his own history - which is tawdry at best.
None of this removes Willy from our sympathy though. He remains a character, a person, who we can relate to, perhaps because he is so deeply flawed and so rawly desperate for affection and respect. He stands at the feet of the audience and begs just to be noticed so that his life might have some meaning, if not honor.