PC makers like Lenovo, Asus, and MSI have all opted for Windows-based handhelds so far, but as consumer patience with Windows on tiny screens wears thin and Microsoft’s progress in improving the experience is slow, it seems like OEMs are looking for a better alternative.
Microsoft hasn't make any progress on improving the experience because the Windows UI is simply not suitable for non-mouse environment, and you are not allowed to sell a Windows device that doesn't provide the Windows OOB experience.
Linux is the only sane choice with restrictions like that.