Taqumi, I think more serious Japanese conversation goes on to the message board rather than blogs. I get this feeling that Japanese people like to keep their blogs casual but don’t mind discussing serious stuff on boards like ni-channel etc.

Few other reasons that I’d personally like to add:

One is the literacy rate in Japan which is virtually 100% making writing to express/note whatever their thoughts easy for anyone.

Another aspect is the internet infrastructure in Japan including both broadband and mobile phones. Cheapest broadband in the world, and heavy mobile phone usage for browsing the internet give you a very good environment for easily start and update blogs.

Though, as many Japanese bloggers reckon, the quality of the Japanese blogosphere is nowhere as near as some form of journalism.
Most of them are just personal diaries. In fact, the word “burogu” (blog) is usually referred to as an online diary in Japan. (possibly another reason why so many Japanese write them even without much contents inside.)

So I would be very careful to perceive this result as an indication that thoughtful, insightful communication is happening more in Japanese blogosphere than in English one.