Promoting someone within organisation is a good idea sir if you are conducting interviews for this particular position it means you require an individual from outside your organisation, otherwise I won't be sitting in front of you for the interview and secondly a new person from other organisation always brings difrent skills and market knowledge which contribute in organisation growth.
People within the organization might have a better understanding of the organizations business, workflow, process etc. And the improvement areas of the company but recruiting a candidate from outside will have a different perspective towards things which provides a chance to implement new traits which he/she has come across for the betterment of the company.
Promoting to existing employees is a good and positive approach, but combination of old branches and new leaves always make a better tree. So it will be always better for a organization if a fresh new attitude/thought mixed with an experience thought.
The company might be thinking that Old broom knows the corners of the house but the new broom cleans the floor better. I'm sure you are looking for a combination of both and that why I'm getting interviewed today.
Why do we upgrade to new technology when the old one is still helping us. Same way we need to upgrade human resources for betterment of the organization and to bring in new ideas.