Work-life balance, favoritism, clique culture.
In my experience as an internal employee, I witnessed that those who are proficient at stroking the ego of leadership or were chosen as a favorite by leadership are the ones who benefit from career growth. It is not based on skillset, but rather if your manager does not feel threatened by you. Managers I have come across will put their own growth ahead of yours, and you will not be allowed to grow before they do.
Advice to prospective internal employees - if you have growth aspirations, especially interest in consulting, make sure your manager has googly eyes for you within the first year. If you do not see this in a short amount of time, you will not be allowed to reach your goals.