Why won't Happy go out West with Biff, and why won't Biff stay? Why doesn't either of them get married and settle down?
The answers to both questions lie in the complicated relationships between sons and father. Happy, as the younger son, has grown up in Biff's shadow. He feels a deep-seated need to prove himself to his father. Hence, he won't go west until he can achieve his father's approval. Biff can't stay because he has no respect for Willy. Ever since he discovered his father's affair years ago, Biff harbors a hatred and disgust for his father which often erupts in anger. It also leaves him unable to be satisfied in any job or location for very long: thus, his restless wandering.
The second question is a bit more difficult to answer, but falls under the same reasons as the first. While we can't really answer why someone doesn't get married (I mean, it's not like one can walk up to a stranger and marry instantly), we can trace the problems in Biff and Happy's relationships. Happy is a notorious adulterer and womanizer. Biff never mentions a girlfriend, and seems uncomfortable with the idea. Both of these problems trace back to Willy's relationship with Linda. The sons have seen their mother bullied, emotionally abused, and ignored their entire lives. Biff also knows his father has cheated on his mother. So there's definitely unresolved issues in dealing with their parents' relationship. This is the most likely reason that Happy and Biff cannot "settle down".
Willy has high expectations and interfears in Happys and Biffs relationships. Biff sees things in reality of Willy's life and doesn't want to be around it. It wants to be able to work outside on a field with his shirt off. Happy is content with having the life that Willy has set for him. But in reality neither of themhave a job and Willy is setting them both up for failure.