Between a rock and a hard place
MS' problem is that phones/tablets have taken up a large proportion of the populace's mindset, as well as cash.
Leaving it to apple and google and valve would lead to losing the home market. That means fewer windows devs growing up with windows at home, which is not an option.
Being fourth in the tablet/phone market is hard to fix, so why not leverage the desktop monopoly to accustom users to the phone/tablet OS? That seems reasonable.
However, the tablet/phone market is not like the PC market. Document formats which provide desktop lockin for MS aren't really used there, so loyalty to a brand/vendor is slim beyond the fanbois. The documents of the tablet are in h264, epub and mp3 formats, so even if people become familiar with the GUI (whether they like it or not) there's little to hold them there unless it is actually "better."
What MS should have done is build a private cloud server for the home. Make information available online/offline and sync'ing brilliant. They should have paired a very low power server with their phone. They should have built a private cloud and phone with no advertising or data slurping. Produce something better, not just rely on leverage or monopoly to work for you. Got a hotmail account? Get fast flux DNS too so people can run drop-box or lync from their own homes without needing apple or google's cloud. Don't annoy OEMs with surface, work with them to provide in-box NAS solutions. Put windowsRT on a PCIe card with SATA and NIC ports to convert DAS to NAS so that when you power off your PC, your data is still all available.
Just like the old windows-on-a-phone attempts, MS has just tried to re-skin what they already had, rather than provide something new and better.
MS is providing a solution (OS) for a problem that doesn't exist. How many people are looking around thinking, we really need a windows OS on a phone?
What people might be asking is, "how can I get better privacy while using my phone?"