Interviewer seeks to test your level of interest in the organization. If he had the intention to promote someone from within the organization to this post then, in the first place, you would not have been sitting in front of him here. The decision to hire an employee and conduct an interview comes after considering all the aspects of the situation. The whole process of recruitment including the interview imposes a considerable amount of cost to the company.
Hence, give a smart  answer to the interviewer. Display your boldness as follows:
  1. Mention about the skills  which shall come along with you into the organization. This shall have greater  impact especially when the job-profile requires a new skill which has come into  trend recently and hence there is a possibility that none of the current  employees of the company shall be proficient in it; whereas you have been  trained in it and even possess a working experience in the same.
  2. Speak about the experience you possess of having  worked in the job-profile for which you are being interviewed. 
  3. Tell them that ‘new ideas and creativity always  adds value to an organization’. Hence, your addition to the organization shall  be fruitful in the long run. 
  4. If you are coming from a background, the knowledge of which shall help you  give a different aspect to the organization, then, do mention about it.

Now, it’s time for 'participation':
*Assume that you are sitting in the interview room now.
*You have been asked to ‘Why should I hire you from outside when I could promote someone from within the organization itself?’. Go ahead; give it a try on how to answer the same. Post your own answers and get them reviewed .

Why should I hire you from outside when I could promote someone from within the organization itself?