I think that whoever made twitter wanted to make it short and sweet. People log on and "Tweet" small tidbits of there life of vent on any subject they desire.
You can only post tweets of 140 characters, as mentioned, but there are tools that allow you to tweet longer than that. Embedle is one. There's also Tweetlonger or Twitlonger, whichever it is. The character limit on Twitter is good to have because it promotes quick, bite-sized content, and also for mobile devices. I'm not sure about now, but Twitter used to be used with text messaging a lot.