Why can't you post a response to the best answer? Answered

The Robot knows. Mere mortals don't.

I suspect it's just an accident of how Instructables evolved, and nobody's complained loudly enough to convince the admins that it's worth changing.

I can make some guesses about what the programmers were thinking, but they'd just be guesses based on how I'd implement a site like this.

You could also ask why only the top-level answer and all the responses to it, rather than a specific reply, can be marked as best answer.

So that spammers and trolls can't "hijack" the thread which is tied to the top of the page. Best Answer is a special case of "featured" or "sticky" comments, which are automatically put at the top of the I'ble, forum topic, or Question with which they are associated.

Actually, its a deliberate function, ostensibly to stop "trolling" I posed virtually the same question you asked in the forums a while back  HERE.

See kelseymh's first comment in the thread.

The world may never know. Or maybe it's just computer goblins! Worse than trolls!!!! Ah!