There may be less poor people in college, but it can be easier for poor people to pay. There is sort of this no man's land where a person (or their family) makes just too much for assistance, but not really enough to pay. My family fell into this area while in college, and I worked 20-30 hours/week on top of a normal 12-15 hour course load. To make up when I took 12 hours, I also went to both summer sessions each year. It was a long 4 years as I had maybe 3-4 weeks out of school each year, and zero time out of work.

Low enrollment is not because of ability to pay tuition for low income. Many poor people don't enroll because they didn't have encouragement from family or they had other obligations (e.g. taking care of family members).