I would guess that the WPF team have all been working on WP7 and now that it has been released, they are concentrating on it again.
Their ribbon came 2.5 years too late, and I have worked on a few projects that opted for poorly implemented versions, where the implementors thought that it was most important to have a ribbon, rather than something that led to maintainable codebases.
I am currently working on a scientific research application, and we needed a WPF chart and guess what? Like the Ribbon, the WPF chart has been beta for a year and a half now, so there is no way we can use it. What is worse is that all the major "partners" cannot create a WPF chart that renders 2000 floating points of data in less that 5 seconds. I have tried them all and they are slow so we have had to regress to a winforms version that renders the same data in milliseconds.
John Papa recently showed some awsome Silverlight themes that ought to be available for WPF, but, guess what? It will be another year or two before these themes arer availalble.
They really need to sort out the end to end availablity of controls for WPF as Silverlight. I am seeing more and more projects at the moment using WPF in research and financial sectors, but developers are having to fight the technology at times, and most of the third party tools suck, including Devexpress whose WPF controls like charts and grouping grids are piteously slow.