It would be nice if a single operating system would fit the needs of most data centres but it never does. Often politics and cost will influence selection but in some cases you get caught with little choice due to application dependencies.
You don’t often see many environments that run without Microsoft Windows (tied by the Microsoft Applications which are only supported on Windows for obvious commercial reasons).
In the Linux space Red Hat and SUSE primarily battle it out and I have come across several applications that do not offer mainstream support on one or the other. As an example at the time of writing HP Server Automation (formerly Opsware) is only supported on Red Hat.
Unix has three notable flavours: Solaris (runs on Sun SPARC but also x86 servers), HP-UX (HP Proprietary on RISC/Itanium) and AIX (RS/6000 and System p, BladeCenter JS-series, IntelliStation POWER, and System i5 platforms.)