I don't understand the rationale for "micro-blogging". Why should a site limit the length of messages to an unreasonably low number of characters, say 200 or 40

I don't understand the rationale for "micro-blogging". Why should a site limit the length of messages to an unreasonably low number of characters, say 200 or 400? Why? I understand there was a case for that in mobile texting because of technological costs and the ability to charge more for more messages. But in the web, why should you put an artificial very low limit on how much people can say? And as to Twitter, why would anybody prefer a platform where you can't even edit your messages?