Nebraska (in their eyes) needs someone who's going to get them to 11-1 and 12-0 type of stratosphere and nobody will be able to do that.
The Nebraska of the late 90s had a ton of recruiting advantages - mostly TV and NFL pedigree. Now, every team is on TV every week. And talent evaluation is so good, the NFL will find you if you are good. Nebraska of old is dead. The best they can hope for is Iowa-type success.
Tennessee is in the same boat. This ain’t the 1990s anymore.
Campbell can accomplish the same things at Iowa State that he can at Nebraska. And probably has an easier road to conference titles and potential CFB playoff berths. Which is why, in my opinion, it takes a Top 3-5 CFB job to lure him away. (Assuming ISU keeps his pay competitive as I expect JP to do).