User blog comment:Sarah Manley/A Facelift for the Edit Page/@comment-13301-20110415201633/@comment-126761-20110415233738

Even if we could represent as much as WYSIWYG as possible (which is something we're working towards), there are always some things we cannot easily show - for example, templates/code that define basic layout or styles, page flags, and comments. So preview always has its place. Besides, making it flash in and out of existence as you switch between source and visual mode would look odd.

Edit: actual reason we moved away from WYSIWYG - it's a relatively obscure acronym: not really appropriate for many people.