In my opinion, and it is just opinion I have no facts to support this at all, if Microsoft released a Linux it would become a nearly instant success in the corporate market.
Year -1 (Now)
System Administrator: blah blah blah so you see how Linux would improve our productivity PHB: No way. We're not having something put together by a bunch of hackers
System Administrator: blah blah blah so you see how Linux would improve our productivity PHB: Hmmm, OK, as long as we do it quietly. To protect us we'd better be safe & go with Microsoft Linux
System Administrator: blah blah blah so you see how Linux would improve our productivity
PHB: Good thinking. MS Linux gets great reviews in PHB Weekly. Just make sure you get service pack 6.2.
PHB: The CEO wants to know why aren't we running Linux on our servers?
System Administrator: It's too unstable, Microsoft keep screwing up the updates.
The few PHBs that ever knew there was a Linux before MS got in the market would quickly forget that unpleasant fact. If they ever heard of them they'd probably think Debian SuSE & Redhat were either cheap clones or outright warez. In either case something to be avoided.
Although I'm glad they didn't, I've never understood why Microsoft haven't done this, it's well within their capabilities.