Third party software support typically only supports the large commercial distros, like Redhat and SuSE (and by proxy their derivatives).
Established support, in the case of SuSE and Redhat.
Tools that are not available on the free versions of the operating systems that allow for easier management of large install bases.
That at least explains Redhat and SuSE, with their free variants getting some of the benefits by virtue of being derivative.
Also bare in mind that "frequently updated" is not always a good thing for large enterprise installations as it means a tremendous amount of QA. Stable tools with predictable release cycles is often preferred over cutting edge when it comes to thousands upon thousands of servers.
There is also a bit of a "traditional" way of thinking in installing Redhat, it's been the major player in enterprise linux for so long and one could easily some it up as "No one has gotten fired for buying redhat."
Easier for the end user is not always easier for the administrators.