I would think she is shy, and may have not been asked out that much because guys are intimidated by her. It also depends on her age. If she's a teenager than I would think its because she's just young. If she's in her early-mid twenties or older than I would think it's because of the first two things I suggested and also because she has a good head on her shoulder and she realizes that she doesn't need a man in her life to make her happy and she wants to have a successful life and move up in the world, and puts her career before a man.