There are blogs which are really just platforms for lots of videos but most blogs use a combination of text and video. Since they could end up as just one line of videos, they need some text to explain, or at least title, each video so that the visitor has some guidance as to use.
Server space. Obviously, with lots of images, vlogs require more server space than text. They also need a faster server as people will not sit and wait more than a few seconds to see the content. But they can still be sent through RSS to keep people updated.
