I used to say why there are only 140 character when i tweet it should be more (Human wants are unlimited) but then i realize that you can be powerful in 140 characters of twitter - it's as simple as you can speak 5 powerful works to convey your message - being philosophical.
It's actually Twitter send tweets from Mobile phones since people can send SMS'es in text with 160 character twitter consider using 140 characters only to send tweet to your followers.
Apart from this there are various other reasons explained by professional with 140 characters limitations of twitter. But a tip here while sending information you may use a URL or source of your information.
For this many short url services are available to help creating long url's into short one of them widely used are ad.fly and bit.ly
Note : Whenever you tweet - try to tweet in less than 140 characters so that there is a space for replies or @Retweets