Walsh was an exception.
Very few assistants stay assistants by choice.
The reason they remain assistants are varied. Some are bad learners, others can't comprehend the complexity of being head coach, wheras some simply can't handle the pressure. I'm not suggesting these are the only reasons.
However, those that assume simply because someone has served a long adjective, or have been an assistant in particular programs, that they would make a good head coach, are sorely mistaken.