US News and World only ranks about 100 graduate programs in computer science 300 research universities in the US. How do graduate programs of small and/or unranked universities 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.
I would think it is a matter of differentiating your department from those at other universities in ways that you are capable of. Perhaps you don't have the reputation and resources to compete with large programs, but are there other areas that make you unique? What makes your program special? What does your department/university/locale offer that is different than other places? Is it a particular sub-specialty? Specific industry-contacts? Killer internships?