Support. Commercial software packages that you may want to run often come with a list of supported operating systems which usually only includes RedHat and maybe SuSE. Ubuntu and Debian are rarely officially supported. And although you usually can get things to work somehow, your application software support will be useless because all tickets get closed with "unsupported OS, use RedHat". With CentOS you at least might have a chance to get a non-useless answer from you application's support team.