it was cheaper to sign a bunch of assistants for a few million. The fact that he admitted that he had to make decision (hidden clue in the interview) means that Kelly was on the blocks. The fact that he clearly forced changes upon Kelly, means that he's not satisfied. There was a rumor that Kelly balked at a reduced buyout.
That said, hiring any new coach would come with significant risk no matter who does the hiring. I don't see any coaches (outside of the big 4) that have less risk than Kelly did. It could be a combination factors... firing a coach is not a solution, it just gets rid of the immediate source of angst.