Since blogs, or online journals, give writers a chance to talk about whatever's on their mind, there are hoops a company has to jump through in order not to offend its customers. Plus, someone on the corporate level such as a CEO or chairman would have to have every word approved by his or her legal department and the public relations department.
From the PR perspective, you have to watch what a top exec says on a normal day and be ready to put your spin on it at any given moment. But if it's written down and distributed to the entire world, you have to make sure the words don't give away corporate information, offend anyone or could be misconstrued into something that's detrimental to the company and its image.
Over time, top CEOs may take on the blog challenge but it's going to take a huge team effort to even get one blog entry posted on the Internet.