Well, the comic above is just a joke. But the real reason was…

Originally, Twitter had no character limit and several issues were aroused. For example, users tend to compose tweets of over 160 characters (the default SMS carrier limit).

As a result, they were split into multiple posts and delivered (somewhat) sequentially, which didn’t look too good. In the end, the folks at the San Francisco-based micro-blogging company decided to place a limit on the number of characters that would go out via SMS for each post. They settled on 140, in order to leave room for the username and the colon in front of the message.

Comic via Geek And Poke