You might want to use MSSQL as a database and be able to sell your product to companies that do not have Windows servers around (and no staff with experience running them). And Microsoft wants to allow you to do that so you don't start building integrations with other databases to support those clients, and then those take MSSQL sales away.

And even in companies that have Windows MSSQL on Linux might be useful, e.g. if they have a linux-centric virtualization infrastructure.