Easier to manufacture, and much more predictable parts content. Apple is in control of 100% of the core OS. They may have to wait for some third party stuff they want to bundle on the CD, but that becomes an easy decision -- ship the OS, then press a new version if any third-party stuff was missing.
I'm certain OS X will be in stores on the 24th; I'm holding out hope for a midnight launch party at the local Computer Store (it's real name; Mac only dealer since the Apple ][!).
-rob.