Probably because:
* SQL Server is proprietary
* SQL Server licenses are expensive
* SQL Server runs only on Windows (or least used to?)
* SQL Server is proprietary
* SQL Server licenses are expensive
* SQL Server runs only on Windows (or least used to?)
Customer: "How much does an Oracle database license cost?" Oracle Rep: "Well, how much do you have?"
To your own points:
SQL Server is as proprietary as Oracle SQL Server is cheaper than Oracle SQL Server is being ported to Linux in 2017 :)