It's "legacy" because it's essentially tied to Windows. Yes, technically it works on Linux, and no doubt that was an amazing feat, but no serious company is running MSSQL on Linux when all the documentation, all the best practices are all based on running that on Windows.
Why did they port it to Linux?
Knowing nothing about this, I wonder if they're getting ready to retire Windows Server, and wanted to get their server products off it?
Edit: How they did it is also quite fascinating:
https://www.microsoft.com/en-us/sql-server/blog/2016/12/16/s...
https://www.microsoft.com/en-us/research/project/drawbridge/
>a key contribution of Drawbridge is a version of Windows that has been enlightened to run within a single Drawbridge picoprocess.
MSSQL on Linux only seems to use parts of that project (a smaller abstraction layer), but that's still super cool.