I suspect that Microsoft doesn’t want to replay the 'Hey, let’s write off nearly a billion dollars worth of Surface RT tablets!" fiasco; but the fact that they went back for another round of Surface RT after doing that (and the fact that they licensed Windows RT to Nokia) suggests that they would rather have more Windows RT units in the wild, at the expense of selling fewer of them personally, than they would have 100% of a potentially-doomed market. (which is not out of character, given that nobody is going to play in the ‘Windows store’ if there aren’t any customers, and Microsoft is no stranger to bleeding cash in exchange for marketshare, see Xbox 1…)
And, given that Nokia has shown a fairly robust capability to design hardware, they probably aren’t a bad party to give it a go.
