Another reason weblogs will remain a niche. the mainstream: the mainstream is what comes to you. Weblogs, at least good, worthy ones, do not seek out an audience. Seeking an audience weakens the integrity of the message. You must seek the message. Thus a good weblog will always be underground. Because let's face it: people are lazy. If you're looking for other people's opinions, you're already way ahead of the curve. You want more than the facts. You want perspective, analysis, personality. You get none of those things from the news, and that's the way most people like it.
Incidentally, the above links are handy. There's this guy who apparently really likes raves. And, apparently, he really hates ravers.