Because a lot of enterprises running Linux are running some kind of "enterprise Linux" e.g. Red Hat, Ubuntu Advantage, SUSE Linux Enterprise, where purchasing an "enterprise Linux" allows for support contracts etc.

If you imagine WSL for Windows Server not as competition for CentOS and Debian, but as competition for "enterprise Linux", then you can have a compelling sales case for enterprises which are already heavily invested in Microsoft, i.e. with extensive Active Directory and Office 365 deployments. Why purchase two support contracts, one from Microsoft and one from Enterpise Linux Vendor X, when you can buy just one support contract from Microsoft and still run everything you need?

As far as I know, WSL is "Ubuntu on Windows", so wouldn't this be a conflict of interest? Maybe they would simply work out a deal to pay a royalty to Canonical or something. Wouldn't that be poetic.

I think it would be really interesting if MS competed in that space. But 'nix sysadmins don't want to worry about maintaining a Windows instance concurrently with their server instances (assuming the only facilities they need are what 'nix provides). Microsoft having Windows attached to their Linux distribution is a net negative, in my opinion.

Edit: Formatting

For the same reason people use OS X to actually develop applications that will actually run on GNU/Linux.

I don't know why you are being down voted. I switched to OSX to get the sweet NIX underbelly years ago and while not everyone* uses OSX for linux deployments... many, many developers do.

So you're saying develop on Windows Server and deploy on Linux? Why not develop on desktop Windows?

No, develop on Windows and deploy on Windows Server with the new POSIX subsystem.