I think another issue facing coaching turnover is the coach's family. Coaching any sport nowadays requires the whole family to be on board. I am a young(ish) coach with 2 children under the age of two. When looking for a coaching job, I immediately ask my wife what she thinks about the possibility of moving to the community. My wife has some flexibility with her job, but we can't just pick up and move. My wife is very supportive and wants me to advance in coaching, but she understands how political some school corporations can be.
Everything Driven T brings up is valid. I have been a part of a coaching staff where there has been poor admin support. This has affected coaches drive to coach, wanting to stay in the profession and time which some coaches put in. Currently I work in a school which does not have football (so I coach at another school), but every time the administration hires a new teacher, coaching any sport is not a conversation to be had. Some schools are simply devaluing the importance of extra-curricular activities. It is a sad state of affairs for some schools in Indiana.