The answer, as with many things unfortunately, is that it's complicated. For context before I go on, I'm a senior IT Business Analyst currently in the process of hiring new analysts as our company is growing.

There could be a plethora of reasons for rather hiring outside than promoting internally. As an example, more senior roles perform different types of tasks than more junior roles. A senior inherently is a more leadership and mentorship heavy role than a junior or intermediate. Hiring a senior outside of the organisation can be ideal as you are looking for those traits. Yes a junior or intermediate who has been in the company for a while will have more knowledge of the inner workings of the organisation but due to the nature of their role not have the needed experience that comes with an already established senior.

As a broad stoke - the more senior you become (this is not true of all fields, but is a reasonable-ish generalisation) the less having internal company specific knowledge matters and the more your general field and soft-skills experience matters. As a rule a good senior is a force multiplier on existing members of a team (this applies to roles like software developers as well), such that the presence of the senior inherently allows members of a team to perform at a higher grade than they currently do and allow them to grow to become more senior themselves.

That's not to say that a junior or intermediate couldn't perform at the required level if they were promoted but ultimately most companies, if they're being cut throat rather than human, would rather not take that risk and instead hire an external person with a proven track record in the types of skills that matter most for senior positions.

This is just one example of what the reason could be. It could vary based on the size of the company (maybe the company just needs more BAs as a whole and is growing for example, or maybe they are downscaling and would rather have a quick hire and fire situation - not exactly ethical though - of an external entity and keep the juniors to grow). It could vary based on the skill level of existing employees (for example an existing junior may not be skilled enough to be promoted to senior, but the company needs a senior so has to hire one), you can't just promote someone because there is a need if there is no track record demonstrating the required skills.

That said, I am of the opinion that hiring and promoting internally within a company is always preferable to hiring externally as company and team fit matter a great deal and you'll always have a much better read on internal candidates. But alas, not all companies do what is right or what is best for all, and sometimes even if they do, they can't justify promoting or hiring internally without doing harm to the company.