Because stuff happens.
1. Spouse gets a great job in a different area
2. New Head Coach comes in and brings in their own staff choices
3. You're a rising star assistant who is ready to be a HC
4. Staffs are consolidated from 2 staffs to 1 leaving a group of highly qualified coaches looking elsewhere.
5. Programs get cut all the time-especially men's programs... again leaving qualified people looking for coaching spots.
6. More qualified people than spots. The job pool is not unlimited by any stretch.
It's the nature of the beast in college coaching. Not everyone gets what they deserve-one way or another.