HP (plus a few others) etc tried it, had some good ideas, but it didn't really get legs.
Apple stripped it down, locked it down and made it look good and it just works (so I'm told).
Then Linux started appearing on more an more units from the more "fringe" OEMs (some iPad rip-offs, some not) and started breeding tablet/tocuh specific distors (Android, MeeGo, Unity etc)
Now HP et al have started to weigh back in with non-MS units.
So MS are not basically playing "Whack-a-mole", the problem for MS is that it's the moles who are holding the mallets in this game. Some of these mole may even be holding nothing more than a sharpened stick, but then know where to jab them.
MS has lost (actually, they never had) the mobile market. It's too fragmented, too varied. Only the ecosystem that Linux fosters can possibly cope with the variations that abound. If MS want to retain any chance, they need to start building cross-platform technologies, applications and services.
This, of course, goes against everything MS stand for. They cannot tolerate (cannot survive) competition on an equal footing.