
I equate this similarly to what happens in non-profit organizations, where an executive may leave and a program director is hired to fill the role. More often than not, the organization and the program director (sometimes) realize that specific management skills are needed that are not present in the program director.
Why do organizations do this? Most of the time it is either out of a sense of loyalty to the program person, who may have been there a great number of years as the #2 or #3. Sometimes it is laziness as going through a search can feel like it will be cumbersome. Overall, it reflects a disturbing view of executive management as believing that these skills are those that can be learned immediately on-the job, which sometimes happen.
The motto is that one position does not mean or equal future success as another. This is also true for those in the corporate community transferring to the nonprofit world. Best way to get there is further education and shadowing before stepping into the role.