Why You Can't Reply to a Sticky Comment or Best Answer
Here's his question:
Sorry to take up your time with something minor, but why could I not post a REPLY to your comment on this Answer, yet I DID have the option to flag it?
(My reply to the question is there, is this not what they wanted?)
When an author selects the best answer, we make that comment thread "sticky" and move it permanently to the top of the heap of other potential answers. You can only select a top-level comment as best answer, but the entire thread travels with that top-level comment. The thinking here is that an entire thread might be the best answer.
Once the answer is marked best and "sticky", we remove the ability to reply to it. This is intended to prevent later comments from being tacked onto the best answer thread, just so they appear at the top of the list. If the author un-marks the thread as best answer, further replies can be added.
The same is true of "sticky" comments throughout the site. When we make something sticky, we feel it's important enough to always be the first comment. However, we want to prevent others from hijacking this position with unrelated replies.