I can imagine the effort of open source Windows would be prohibitive.
Having to go through every source file to ensure there is nothing to cause offense in there; there may be licensed things they'd have to remove; optionally make it buildable outside of their own environment...
Or there may be just plain embarrassing code in there they don't feel the need to let outsiders see, and they don't want to spend the time to check. But you can be sure a very small group of nerds will be waiting to go through it and shout about some crappy thing they found.
I'd venture that even more nerds would go through it and fix their specific problems.
It's always been quite clear that FOSS projects that have sufficient traction are the pinnicle of getting something polished. No matter how architecturally flawed or no matter how bad the design is: many eyes seem to make light work of all edge cases over time.
On the other hand, FOSS projects tend to lack the might of a large business to hit a particular business case or criticality, at least in the short term.
Open sourcing is probably impossible for the same reasons open sourcing Solaris was really difficult. The issues that were affecting solaris affect Windows at least two orders of magnitude harder.
It's the smart play, though they'd lose huge revenues from Servers that are locked in... but otherwise, Windows is a dying operating system, it's not the captive audience it once was as many people are moving to web-apps, games are slowly leaving the platform and it's hanging on mostly due to inertia. The user hostile moves are not helping to slow the decline either.
Having to go through every source file to ensure there is nothing to cause offense in there; there may be licensed things they'd have to remove; optionally make it buildable outside of their own environment...
Or there may be just plain embarrassing code in there they don't feel the need to let outsiders see, and they don't want to spend the time to check. But you can be sure a very small group of nerds will be waiting to go through it and shout about some crappy thing they found.