Asus was previously touting a Transformer Book Trio which ran both Android and Windows 8. It appears to have vanished from existence, and I now wonder heavily if they got a visit from the Microsoft legal or marketing department (as seems to have happened with the Ativ Q)?
I always thought it was because of lacking demand. I'm a former owner of the original Asus Transformer (Android-tablet + keyboard dock) and I still fail to see why I would want to have a device with two architectures, and two OSes. People love to talk about Android and fragmentation, much more so than I find constructive, but putting both Android and Windows in two different universes on the same device is pure madne…
It's pure speculation that it is Microsoft, but it does seem to be a legal problem in the case of the Ativ Q (patents are mentioned but I'm not 100% it is really patents because some sources just say "legal problems"):
http://androidcommunity.com/samsung-ativ-q-hounded-by-patent...
Microsoft is famous for making agreements with OEMs constraining them from shipping other OSes instead of or in combination with Windows. Their motive seems very clear to me in that they would be desperate to prevent Android from rising into the laptop / netbook space. This kind of transition device could be very threatening in that regard.