he'll be hired on the basis that he knows the club, the players respond well to him, he won't have to get used to the way the club operates (like a new manager would). Players trust in him and there'll be minimal change. Even when Pearson has been in charge and when Ranieri was in charge, it was well recorded that Shakespeare would do a lot of work in terms of coaching and managing the team especially during training.
If anything, the fact he started off so well in being a manager just cemented his abilities to step up to that level from assistant manager.