One explanation for this development is of course the ease of use, immediacy, reach, informality and interactivity of Twitter. It possesses all of the key social media super-powers, and it is a little bit better than blogs in all these areas.
To me there is one very important thing that separates Twitter from blogs: it is more democratic. Anyone can join and the barriers to start tweeting are even lower than the barrier to start blogging. Twitter has also room for those who only want to follow and listen or speak. But more importantly, Twitter also provides a neutral ground - I like to think of Twitter as a public square or park where everybody have their own Speakers Corner - where bloggers and readers, who are often the same persons, can meet on more equal terms than in the blogosphere: they are all tweeters and have the same powers in the twittersphere. On a blog, the blogger is the one who is setting the rules and who has the power to silence someone who thinks differently. In the blogosphere, you also need to master how to set up and promote a blog to be heard. There is of course more to it, but I think these things are important when explaining why conversations are moving from blogs to Twitter.