Based on their interview day explanation on how admissions works at this point, I would say that the reason they are stalling on giving out the remaining interviews is because they have to see where they stand in terms of IS/OOS, diversity of experience, etc. so they can produce a balanced, well-rounded class. On my interview day, the director of admissions directly said that as of 1/8/10 they were still not attempting to meet "quotas," but with the later interview/admission dates they would have to start looking to balance out the class. Since the 1/8/10 interview date hears back on 1/14/10, that gives the adcom a few days to better figure out the make up of the incoming class before releasing more interview offers next week.
Good luck to those still waiting!