Paid support is available for MySQL and PostgreSQL too. It is from different companies than those who offer the SQL Server/Oracle/whatever support though, which may a problem if the newcomers don't already have a foot in the door already. Think of it from the business POV: why would the sellers of existing support leave their business to some outsider?
One reason is, Paid support and excellent integration with existing Microsoft products. The features are totally worthy to use SQL server.