Window style with WPF Ribbon from Microsoft doesn't match Windows 8 style
The WPF Ribbon isn't a wrapper for a control offered by the operating system (like most of the Windows Forms controls) but built from scratch to have the look and feel of the ribbon at the time it was developed while being a control that is as flexible as other WPF controls (notice the support for template parts).
That said, it's a matter of the control not (currently) being updated to support the Windows 8 style. Given the number of templates for multiple controls that would have to be changed, as well as the need to be backwards compatible (and this is my speculation at this point) it may have been seen as not a priority when .NET 4.5 was released.
You might be able to change the theme yourself, but that looks like a massive undertaking, so it might be worth it to you to use a third-party control which already implements the Windows 8 ribbon style.