So the obvious answer is because it's only asking for 140 char (i.e. a concise message).
However I think it's also because of
(a) "follower" model in Twitter - that means there are people who I know are going to listen/see my Tweet (assuming they are actively on)
(b) some of the great apps that are easily accessible on my iPhone (TwitterFon)
(c) some of the great apps that are on my desktop (i.e. TweetDeck) that enable me to connect to multiple social networks, and to personalize the searches (i.e. show me the replies to my Tweets)
(d) the fact that I can get SMS to my phone about people's tweets or reponsses to Tweets
(e) the tiny url phenomenom that spawned for Twitter (such as http://bit.ly/) that enables me to shorten urls to letters/ids that say nothing about the actual place it is taking the user and which thereby entices people to click on them (IMHO)
(f) the hashtag (i.e. #Word) is surprisingly something that alot of Twitteres use - in essense this is a searchable tag that is used as a way to make the content easily searchable.
Anyways net net lots of cool (though not necessarily innovative) reasons to want to use #Twitter instead of Blogger.. though not cool enough to get off of Facebook...
Below from www.compete.com looks like sometime in May the crazy onboarding of new visitors to Twitter finally slowed down. Maybe its the sunny weather, or maybe folks have decided facebook has more stickiness (at least that's what the graph shows).
Facinating to see what will happen in the coming months...