Because no one wants to learn something new like postgres, vendors haven't adopted other databases platforms other than SQL, and licensing is absolutely stupid expensive for SQL, so most companies just stay on what they currently run. It costs money to hire new employees who know other databases types, and it costs money to train current employees, it's just an absolutely stupid vicious cycle Microsoft has created. The barrier of entry for new versions of SQL is so high, that it's just not worth the hassle and the price.