I think they ported to Linux because Linux is being used by startups instead of Windows Server and they wanted them to use their SQL Server database instead of MySQL or PostgreSQL so they can sell tech support for SQL Server.

Keep in mind that Microsoft sells tech support to companies with a special hotline they can call and get answers and support for enterprise level software. Porting SQL Server to Linux means more copies of SQL Server being used out there and more tech support phone calls that companies pay for.

With Office 2000 there was the MSDE on the CD-ROM that had a developer version of SQL Server that could run on Windows 2000 Pro to test out databases without using Windows Server. It was limited in number of connections and other things.