I think that money is absolutely the factor, because a high school coach wouldn't always start as a head coach. If JRob were to retire tomorrow, would Minny really trust an untested high school coach to be their head coach? Probably not. Most likely, a high school coach would have to break in as an assistant, and if that coach was making $60-70K as a high school coach and teacher, they won't want to take the inevitable pay cut.
From the coach's perspective, it's also a hell of a lifestyle change and that might not be attractive. I'll use Keith Healy, coach of Providence High School in New Lennox, IL as an example.* Healy has several team state championships, and has several individual state champions. He has kids all over D-I, went to Illinois and still has a cozy relationship with them. I'm sure if he ever wanted to jump to the college ranks, they would find a place for them. But why would he want to?
He is in his late 40's/early 50's, and has a family. He would have to start the extensive traveling that college coaching requires, and that doesn't stop in the off season. Yes, a high school coach's life is extremely busy, but they can sleep most nights at home.
You're also dealing with an entirely different mindset of kids. High school coaches don't get in it just because they love wrestling. You've got to like kids, too. College kids are on a completely different wavelength.
*I am in no way saying that Healy is looking to leave Providence. He was just the first coach to pop into my head.