"Not available yet", at least to me, carries the implication that it will, not just might, become available at some point. Probably soon enough that it's worth asking the question again later. Whereas "don't know" suggests that it's not worth asking the question again, because you're probably just going to get "don't know" as the answer. (At least, if "don't know" and "not available yet" are both presented as options).
A more accurate description is “May become available”, it might be possible that it could become available and we believe it could within a short order, however it is also not assured or guaranteed that it will become available when we think, and it may also never become available at all for any number of reasons as simple as someone not following up and adding newly ascertained information or that the only possible source of the information has become permanently unavailable for many other reasons of their own.