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?