Because all poor people (the white ones too) have real disadvantages. A lack of good schools, parents not able to supply all of the extra things (like test prep), maybe a requirement of work. I would be 100% supportive of need-based "affirmative action", which is what disadvantaged status is. Theres no way in hell I would ever qualify for it, but I have reaped the benefits of being from an upper middle class family.
Now, programs which are geared specifically for African American or Hispanic students are totally unfair. Any "disadvantages" faced by these groups NOT stemming from their poverty are usually either imagined or cultural. I'm sorry, but not trying hard in school because getting good grades is "acting white" is not a good reason for me to feel sorry for you.The success of recent African and Caribbean immigrants who are not part of the prevalent African American culture shows that success is not impossible because they're black. The (relatively more) widespread poverty among these ethnic groups is more of a historic artifact than anything else, since even with America's high social mobility, most people who are born poor will die poor. Theres plenty of programs to help poor individuals though, and I definitely support more.
Also, before flahless brings it up again, there were some other, historic forms of affirmative action. But most of them are no longer relevant. For example, helping women get into med school is no longer an issue as they matched, and even surpassed men in a lot of schools, decades ago. There is also no longer any sort of real discrimination against veterans, who probably now have better shots of getting in than the average person by virtue of life experience.