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By default, blog comments are set to nofollow. You have to install a plugin to make them "dofollow". Many people just don't (and don't know to do that) and some choose not to as it leaks PageRank from the post page with so many outbound links.
Also having the blog set as nofollow cuts down on the spam. I decided to change mine to "dofollow" since I was actively seeking dofollow blogs and leaving comments. Didn't seem fair to me not to allow them the same link love back if they decided the wanted to comment on mine. I woke up today to quite a few more spam comments than normal, so I can see how some wouldn't want to bother turning it on.