While I understand your general view on this, Nebraska's situation was quite different from many other coaching changes because Bo never had a losing season and he always won at least 9 games. Both very rarely involve a coach being fired unless there is some non football related issues going on. Because of this there was a lot of issues with the players truly buying into Riley.
So say Riley comes in without his trusted assistants, then what? You are starting over trying to gain the trust with your coaches and your players. That would be an incredibly difficult situation for someone to be in while also being expected to win (which I know he didn't in year 1 anyway). So he brought in some trusted coaches to try and help change the culture and get the program headed in the direction he wanted it to go. Now after having gotten the culture going in the right direction, he has been able to truly evaluate his assistants. And I think he saw they weren't getting it done, but they did help with the transition.
There's really few situations we can compare this too. Michigan under both Rich Rod and Hoke had losing seasons to end their time there. Bo never had that. He just was good at being mediocre, while getting his players to be loyal to him. So it easy for both the administration and the players to understand a firing when coaches aren't getting the wins. It's not the same when the team and coaches are consistently getting wins, just not enough of them.