Back when it was twttr you interacted with it by sending a text message to its shortcode and it relayed it. Also displaying the messages publicly on the web seemed like a privacy issue to techcrunch of all things. They really caught on by pushing it at sxsw for keeping up with other people there. As for the Ruby thing, I didn't follow it that closely. _why coined the term tumblelog for a blog consisting of really short posts, links and images. Microblogging was the term that stuck, but tumblelog gave tumblr its name. Because _why was excited about the format and he had his weird cult of personality going the Ruby community got excited about it too. Twitter started out as a Rails application, so a lot of its early users where Ruby guys, for the same reason there were a lot of lispers on reddit early on.
Meanwhile, we're all still here because we love SQL