I worked at a dept with ~2.5% acceptance rate. There were two, subtle factors in play: soft ageism and PI's hiring their clones (in thought, gender and race).

I worked at a dept with ~2.5% acceptance rate. There were two, subtle factors in play: soft ageism and PI's hiring their clones (in thought, gender and race). So the dept gravitated to predominantly two minorities, at least in terms of staff, faculty, visiting researchers and grad students.