Beyond other reason listed in the comments, Microsoft is heavily invested in Postgres. They bought Postgres sharding company (https://www.citusdata.com) and apparently it sees a ton of use internally even for newer Greenfield projects. Also, if check recent commits to Postgres, you will find Microsoft employees all over commit history.

As I've commented several times before, MSSQL appears to be on life support. It makes them a ton of money and it's supported in same way AS/400 is supported. Sure, Microsoft will take your money but don't expect a lot in return.