I have read this article from High Scalability about Stack Overflow and other large websites. Many large high traffic .NET sites such as plentyoffish.com, MySpace and Stack Overflow all use .NET technologies and use SQL Server for their database. In the article it says a source in Stack Overflow said:
As you add more and more database servers the SQL Server license costs can be outrageous. So by starting scale up and gradually going scale out with non-open source software you can be in a world of financial hurt.
Why don't these sites use MySQL instead of SQL Server?