Hi Jillaine,
The comment for explaining a merge proposal should always appear when proposing a merge. When creating an unmerged match or a rejected match, it only shows up if you're matching or rejecting a merge that was proposed.
That is, if someone thought two profiles were duplicates and proposed that they be merged, and you are deferring or rejecting that merge, you are asked to explain your decision. If you are proactively going through suggested matches or a list of names, you're not asked to explain it. The assumption is that creating history items and comment postings for these would just add noise. If in certain situations it wouldn't, you could always post on the bulletin board with an extra click.
We could look at adjusting this policy down the road if that's what people would prefer.
Chris