The answer to your question may come from two sources. First, Arthur Miller's Death of a Salesman was written during a time, namely the early 1940's, in which the ideal of getting rich quick and the idea of the American Dream were acquiring strength. As we know, these are the very ideals that Willy Loman embraces: Willy's idea of the American Dream is based on quick success, easy money, and a ready-set reputation once we become "well-liked." Miller infuses the meme of the time with his main character to illustrate the ideals of the time.
The second source of your answer is in the play itself. Your question is correct in stating that Dave Singleman is not the only successful man Willy knows. However, he is still the one who leaves the bigger mark upon Willy because Willy placed a lot of importance on popularity. Surely, he clearly has seen the success of his brother, of Charley and Bernard, and maybe even the success of fellow salesmen. Yet, Willy finds Dave Singleman to be above standard when he learns how Singleman's funeral was big, noteworthy, and that a huge amount of salesmen attend the funeral to pay their last respects. To Willy this was the ultimate illustration of success.
Moreover, Willy's idea of true happiness is to have plenty of friends, plenty of money, plenty of fame, and plenty of power. Dave Singleman represents all of those things. Willy's brother may have become rich, but he is not famous. Charley and Bernard succeeded only to the point of a content middle-class life: That is not enough for Willy. He wants it all. Unfortunately, like Biff says:
He followed the wrong dream
It is obvious that Willy is so beside himself with the myth he has created out of Dave Singleman's life that his dream is not really to be the best Willy Loman he can be, but to be the closest thing to Dave Singleman. He is following another man's path, and living another man's dream. That is why, in the end, he fails at everything.