The trouble is that many lower-income students don't have the luxury of studying an unremunerative field just because they're interested in it. According to this University of Rochester study, many choose a major more on the basis of whether they feel they can earn a living doing it than any intrinsic liking for it. This may be one reason why lower-income students as a group don't do as well in college as upper income ones:
For instance, wealthier students appeared more likely than low-income students to achieve success based on their interest in studying certain subject areas. It's not that low-income students don't want to study various areas, but their motivation for enrolling in college may be more related to a desire to improve their financial situation, and that has a strong impact on their success.