There are a couple of problems from our (Operations) perspective.
1. The infrastructure needs to be supported as well. If the various necessary agents (backups, monitoring, application distribution) only work on Red Hat (or CentOS) then Red Hat is what's acceptable in the production environment.
2. The staff needs to be in place to support it. We have three major Operating Systems we support (team of 5 admins). Solaris, HP-UX, and Red Hat/CentOS. With almost 1,100 systems, environments outside our expertise are difficult to manage. Of those 1,100 systems a very very small percentage are Ubuntu (and Suse, Fedora, Mint) and they aren't supported to the level of the primary systems.