Because USC has some of the best talent in college football who has horrible coaching, and if he can turn that offense around it might leverage him for the HC job for one of the most prestigious colleges in college football, or opens up a HC for another power 5 or NFL team.
It was a genius move by him, and him jumping to an be on an NFL team right now mean they have the pieces to be successful where that is not the case in the cupcake Pac 12 with that roster.
Kliff isn't going anywhere this year, but next year who knows.
With that being said Scott Linehan is as good as gone unless something miraculous happens in the playoffs. Adam Gase did a lot with little talent in Miami, and it would be interesting to see what he could do here. My big issue is he seems to not play some of his better talent as much as you'd want although there might be something behind the scenes there with Drake and Parker.