Had they shipped Astoria in Windows 10 to allow Android apps to "just work" it would have destroyed their UWP strategy.
So they might get a few more Windows Phone users but they would have lost control of the new development platform and given it to Google on a silver platter. And they would never have got anywhere near the numbers Android is at.
I would be shocked if Microsoft don't bring UWP to macOS, Linux and Android (I don't see how they can with iOS) in the not-too-distant-future.