Although this is a worthy topic to explore, the reasons the article mentions are are lousy. So what if there are people making a living blogging? Who cares if Jason Calacanis or Scoble changed their blogging patterns? Who cares about all the noise in the blogosphere, and competing services like Twitter.
The most valid reason is that blogging is hard, and most people aren't willing to put forth the effort to make their blogs readable.