This quote refers to Willy Loman's brother, who found his fortune in Africa in the diamond mines. Willy's brother worked hard and made his fortune early, while Willy worked hard and still hasn't made his own fortune. Willy believes that it is possible to "get rich quick," and it only requires the right circumstances and hard work, but he also thinks that his lifetime of work entitles him to a fortune eventually. He does not fully appreciate the difficulty of working in Africa versus working sales, and thinks that he should eventually have his fortune even if he never moves out of his comfort zone.
WILLY: You guys... There was a man started with the clothes on his back and ended up with diamond mines!
HAPPY: Boy, some day I'd like to know how he did it!
WILLY: What's the mystery? The man knew what he wanted and went out and got it! Walked into a jungle, and comes out, the age of twenty-one, and he's rich! The world is an oyster, but you don't crack it open on a mattress!
(Miller, Death of a Salesman, Google Books)
Willy recognizes that hard work is essential to success, and although "the world is your oyster," you must put effort into it instead of waiting for fortune to fall in your lap. Just as you cannot crack an oyster on a mattress, it being too soft, you can't relax in work, and must continue to strive to be your very best. However, Willy's lifetime of hard work has left him exhausted and mentally fragile, without the fortune he sought; he thinks that he deserves it because he put the effort in, rather than recognizing that sometimes events just fall in a certain pattern. Nobody deliberately blocked his success; he simply moved in a straight line, never falling below or rising above.