because of Oracle...
In many organisations for whatever reason the choice is often limited to MS-SQL and Oracle. Also many applications you can buy support MS SQL or Oracle.
In corporate dollars, running it on Linux can be a lot cheaper. The license costs are not so much the issue. If you have a good Unix team, maintenance costs per server can be a lot cheaper.
And if OS is not a factor, MS SQL vs Oracle can be very tempting. Technically Oracle may be ahead, but their license schemes would make Tony Soprana blush. If you don't need the extra Oracles features, MS SQL can be a very tempting proposition. Lot's of companies would love to leave Oracle because of their license thuggery. Lot's of them don't even run Oracle on virtualised servers due to the stupid licenses.
This means that MS SQL can be a much stronger competitor to Oracle, and that can mean this can be a major cash cow. To me, regardless of good press with the open source fans, MS choice to port MS SQL to Linux may financially be a very smart choice. They may loose a few MS Server licenses, but might end up with selling a lot of MS SQL licenses.