For Microsoft to become a true open-source company they would need to open source Windows, which goes against their advertising, telemetry, and tracking goals.
As sort of an example, MS SQL server is moving to Linux and while not open source, the playbook of eliminating differentiation is at work.
1. It is directly aimed at Oracle and DB2 who traditionally run on Unix environments. It isn't clear that Oracle and DB2 can be as effective in the Windows environment as MSSQL can be in a Unix environment.
2. SQL Server in the bizspark program is nearly free for the first 3 years (so I recall) and then you're locked in and the price jumps substantially.
3. Traditional IT managers tend to want to buy from MS for the possibility of support - support for Postgres via IRC is excellent btw.