We can read here that there is a minimal reputation to comment a post. But there is no minimal to post an answer!
I believe that an answer is more important than a comment.
I admittedly have no idea why this actually is the case, but my guess would be that the reasoning might have gone something like the following:
commenting is perhaps more conducive to spam than is answering, and so we might want a user to demonstrate that they really intend to be part of the community before allowing them to comment wherever they please. Of course, hypothetically speaking, answers are equally good places to post spam (or irrelevant information that detracts from the purpose of the site). I would presume that the fact that answers can be downvoted and have an impact on reputation (and, in certain cases, the user's actual account privileges) is, at least theoretically, a preventative measure.
Of course, this is just speculation on my part. It's an interesting question.