The number of people who might switch from OS X to a mooted Windows with a POSIXy toolchain is approximately zero. Engineering time and energy are not free.

Last time I looked the vast majority of Windows users don't need the console (or even a terminal emulator) and don't tend to compile software themselves or use large parts of the usual UNIX tools. So there are very few users who migrate away from Windows due to that and thus very little incentive to invest a significant amount of effort and money in that.

Then there is the point that Windows is very different from UNIX (even though you can support POSIX as an API layer alongside Win32). As Jeffrey Snover (one of the architects of PowerShell) once put it: They thought about that but noticed that UNIX tools won't help any Windows sysadmin as they are good tools to work with a UNIX system that's mostly based around text files scattered throughout the file system. But they don't work so well in managing Windows which is more centered around the registry and other mechanisms.

Microsoft is an organization made up of many of the smartest, most creative developers on Earth that has been completely hobbled by organizational bureaucracy and office politics. Microsoft stifles more good ideas in one year than most companies have in a decade.

Would you want Ballmer flipping your desk out the window because you dared to dilute the purity of the Windows ecosystem with UNIX?

Thus, PowerShell instead of something like bash.