A lot of these college coaches, prefer coaching in college. Not sure if Leonhard is one of them, but it wouldn't surprise me if he prefers the college coaches lifestyle, as well as the job stability he now has at Wisconsin. I also don't think the money at the Pro Level is necessarily more and in many cases, its less. As someone mentioned, I think Leonhard is rignt around $1M at the UW, while the NFL doesn't report coaches salaries, I read that DC's are right around a high six-figure to low seven-figure yearly salary.
I see Leonhard following the same path as the guy he replaced, Dave Aranda. Aranda was a highly successful DC at Wisconsin from 2012 until 2016. He came in and replaced the Badgers 4-3 with a 3-4 and had huge success. Aranda then made a lateral move to LSU, at a much higher pay and is now the head coach at Baylor. Rough start in his first year at Baylor, going 2-7. Hey, maybe the Packers can hire Aranda, Baylor is only paying him $4M/year.