I left the Windows division shortly before Vista shipped, so I couldn't say. But from native code, the choices were to do things directly on the win32 API or to

I left the Windows division shortly before Vista shipped, so I couldn't say. But from native code, the choices were to do things directly on the win32 API or to use DirectUI which, for all its faults, was much easier to use, and produced much more modern UI/UX. There were also the obvious advantages of everyone using the same technology. At the time, the look and feel of a windows release was a closely guarded secret until fairly near the release, so the team that owned that experience needed to be able to apply that theme to everything fairly quickly.