[jasomill]

This, and it may have also been a legal thing. "Product for Third-Party OS" has been accepted as a descriptive use of a third-party trademark for decades, requiring only proper attribution rather than a license, whereas marketing a product that didn't even originally use the Linux kernel as a "Linux Subsystem" might have been considered riskier by Microsoft's lawyers in spite of the nonstandard use of the former.

[al_borland]

It could be even more simple. Microsoft would want to their own product, Windows, to come before Linux in the name.

I read through the brand guidelines where I work, and we have a similar stipulation. Maybe there is some law mixed in there, but from a pure branding play, a company will never want to put someone else first.