1) Redhat has a long supported lifetime for each distro (e.g. rhel3, rhel4, rhel5...)
I think RH distros are released every couple of years and then highly supported for 4-5 years, then supported for security and major bugs until 7 years, then finally End of Lifed.
This long life cycle is valuable for companies who develop software as it means you can run your software for up to 7 years without having to update it a lot, you know APIs won't change, defaults probably won't change and major versions won't change. However this is also a major gripe for some, especially developers).
However security and bug fixes will be back ported and on occasion new features.
New packages may be introduced e.g. a new interpreter might be introduced if it is becoming fashionable.
See the following link for details:
2) A lot of commercial software is released for rhel.
In fact I'd say that it is the most commonly supported linux platform for commercial software. It has a lot of reach with the large scale enterprise apps e.g. oracle, peoplesoft, sap, db2 are all supported.
3) Same with hardware. Most if not all tier 1 hardware vendors (and even some of the dinky little HW vendors) provide drivers, firmware installers, utils and config tools for RHEL a well as certify their hardware to work with RHEL.
4) It has good management software for small and large scale fleets i.e. RHN and Satellite server (basically your own local version of RHN).
5) RH provides indemnity against patent infringement cases e.g. SCO style cases
6) RH provides support. Possibly the best linux support around. Not sure. However I've found the support to be lacklustre myself.
6) It has training and certs - both the training and certs are good from what my colleagues say. I've never attended any, but my colleagues, who I respect a lot, all say good things about the training and all think the cert is worth something because the exams are practical and difficult, even for god, experienced sys admins.