Because Twitter is brand new. That is one factor but truly let's take a quick look at the evolution of personal communication and the psychology behind it. Cell phones, besides being huge, were once exclusive. Only people who needed to be reached at all times would have one. The same with pagers before that. These days, one wouldn't leave the house without a cell phone. What does that mean? Well imagine going out before we all had cell phones... Now think of leaving the house without your cell phone now. What if something happens? How will I be reached? Well how did we manage without cell phones? (a lot of questions for an answer, I know).
We're all about reachability now. Being able to get in touch with people at anytime, one way or another, passively or actively. See, the burden of being reached on a cell phone begats the ease of communicating via text. Now with Twitter you don't have to actively communicate with anyone; you can just post your status updates! People can check you out, know where you are, make plans for you and what have you. We're addicted to keeping track of each other whether it's Myspace, Facebook or the easiest tool for keeping track, Twitter.

From personal experience, once you have found people to follow, and people that follow you, you feel as if you are a part of their lives - only a close friend knows the sort of things most people broadcast on Twitter. Jonathan Coulton is in an elevator in Chicago! Leo Laporte is streaming live! Once you feel like you know people, you feel close enough to interact. When people respond back, you start whole conversations, and feel as if you are in a community like any other internet community. The more time you spend with Twitter, the more personal it becomes, with a "friends list" people who have actually become your friends.

Twitter actually has some nice points about it as compared to the other social networking sites. Facebook, MySpace and similar services are really geared towards the young 'uns who have *time* to be tweaking the look of their site, playing around with the gadgets and plugins, and doing the endless micro-management that can be done there.

Twitter is simple ..which makes it flexible and powerful. Tweets are limited to the size of an SMS which allows you to use it from any cellphone. The size has an additional benefit of lowering the signal-to-noise ratio. ie: tweets are always short, and to the point.

Its flexibility lets you use it to keep in touch between friends, within an organisation/business, or to keep up with current events, news, or trivia from any blogger or organisation you choose. ..and you decides who you follow, what content you receive, and most beautifully there's no spam because you only get content from those you follow (no content is ever forced on you from unknown people.)

Once you're on the Twitter is can feel quite addictive, because you're getting a continual stream of information, whether it be plain conversation, announcements, trivia, or news. You're never alone. :) And the things you tweet can be to specific people or to all followers ..getting the word out is easy.

So the way it's so quick to set up and low maintenance to use, Twitter has both become popular, and aquired an older demographic than Facebook/MySpace/etc. Twitter's main (technical) problem now is that it wasn't created with a "push" notification model, so they might find it difficult to scale. If it entirely falls over due to it's own weight, I expect the open-standards that XMPP/Jabber are based on will fill its space. (Much better architected, and you can run it on your own server if you choose, so businesses can use it without the security risks of Twitter.)

1. It gives people something to do when they are bored
2. People believe that everyone wants to know everything that they do all day long, even if that's not the case (ex: I'm making a sandwich right now, can't find the lettuce!)
3. It's a cool application for people to communicate over... kinda like a chat room that you can answer at any time
4. People get addicted to anything that they can spend hours doing on the internet

Anytime I think of people addicted to twitter, this automatically comes to mind: http://www.penny-arcade.com/comic/2008/4/23/ (note: includes some language)