Because NT, as in modern NT, is pretty damn sophisticated. Switching to Linux really wouldn't be a step up for Microsoft.
As I said before, it's the crap that's been bolted on that really makes Windows feel like a dinosaur. It's the decades - literally decades - of backwards comparability, the lack of a proper package management system and the free reign that developers gets that makes Windows a mess. It's the multiple shells that try to coexist or the feature bloat of pass releases or even Microsofts past tendency to reinvent the desktop every few years rather than introduce incremental improvements. This is all issues independent of the kernel, syscalls etc - i.e. All issues Microsoft would have to address regardless of which core they chose to run. So switching to Linux wouldn't help them. If anything it would just make things worse because they would forgo the stable graphics ABIs in NT for the uncertainty that is Linuxes display server wars. They'd forgo their stable Win32 APIs and other libraries for WINE. They'd have to reinvent DirectX - who's support in WINE is several generations behind - and have to battle with the appalling state of Linuxes graphics card drivers. And that's just scratching the surface of the problems they'd face.
I do love Linux and find Windows to be a hateful platform, but pragmatically Windows switching away from NT would be the dumbest thing Microsoft could do. NT simply isn't an issue.
I also know that I do not look forward to a world with only one dominant desktop/server OS kernel. We need competition here and Microsoft adopting Linux would ruin that.
NT might be decent but it's not great. Why waste resources on it when they could be put to more lucrative use?