The overwhelming majority are (at least in the US), primarily because Redhat will actually work with upstream projects and fix bugs. Canonical will say, "Lets see if Debian will fix it for us".

One of my employers spent a multi-year effort to replace Ubuntu entirely (with RHEL) after Jane Silber came onsite and agreed that their support organization could not work on the things we'd wanted them to, because Redhat was already the upstream working on those things or the preferred partner of said hardware vendors. This isn't heresay, I heard it from her myself (maybe 4 years ago?).