I have figured out why it's better to have blog posts arranged so that the reader is faced iwth the most recent at the top, while comments should be arranged in the opposite manner, so that readers have to scroll down to reach the most recent.

Because blog posts are generally individual reading items. They may relate to a former post, but more often than not, they don't. So there you are, you don't need history, you are looking simply for the genius inherent in each tiny morsel of wit.

Comments, on the other hand, are a discussion between the writer and the readers. Most often, each comment is a reaction to the one before. The more comments, the more likely this is to happen. Therefore, it is best to start out with the oldest first, so that the reader can have the background to each comment that is made, rather than reading something and thinking "huh?", and then reading farther and finding the basis for the remark.