How do professors from very small or unranked universities in the U.S. attract graduate students?
Not everyone has the GPA / research experience to be accepted to those top 100 Universities.
Not everyone is confident enough to apply to those top universities and they feel more comfortable going somewhere closer to home.
Many students have constraints making relocation impossible (e.g., cultural obligations to their family where they cannot move more than 10 miles from their parents), so they go to graduate school in the closest program they can.