Helen de Cruz has a great post up at NewAPPS that discusses, among other things, why graduate students might opt to attend unranked programs.
Another, often overlooked, consideration in play for some graduate students is disability. Some campuses are more friendly and accommodating to students with particular kinds of disabilities, some local communities have more resources than others, some states have policies that make it easier to be funded by vocational rehabilitation than others, some states (in the U.S.) provide tuition waivers to students with certain disabilities, and so on.
Feminist Philosophers 