When you come from a lower income you are quite possibly the first and only person from your family to go to University, or maybe even finish high school. Even after doing well in High School you have no clue what to expect, and no reference points. The idea that even as a graduate you may be paid more than your parents combined is hard to get your head around for doing something you enjoy studying.
Having to do part time work to cover the costs your parent cannot doesn't help, but more than anything else there is the nagging doubt that you shouldn't be there and should go do manual labour and retail like your family was raised to do. You wonder when you will get caught and told to go back where you came from, so you tend to keep your head down and not make best use of the support systems the Uni already has in place for everybody.
You just got to be a stubborn bastard and keep at it, but any encouragement does go a long way since you just don't know how the system works.