Because recruiters are lazy. They cherry pick colleges and build relationships over a period of years. Then they decide that they need more black people, so they send someone to man a booth at a historically black college and expect the same level of polish. (Which is mostly BS)

I had the same experience as a graduate of a mid tier public college. Finally a recruiter from a big company laid it out for me -- they have a list of schools they recruit from, period unless you are referred.