Microblogging allows users to express themselves without the confines of being stuck within the boundaries of a particular topic. Microblog moderation tends to be a hell of a lot more hands-off than forum moderation for that reason. Microblogging does not need to wait for a topic to be approved before it can be discussed, and people are free to follow microbloggers for their insights in a way that doesn't really exist in forums.

Forums are walled gardens. Microblogs are blank canvases.

We don't need a "forum protocol." We have Usenet.

Also, many people don't really want to have in-depth discussions with their friends on the internet. What they REALLY want to do is follow famous people and intellectuals and share their thoughts to THOSE people in the hopes of getting noticed.