People who are using commodity stuff probably moved on. People wanting to extend RHEL 3 support into the future probably have custom/proprietary software that is too costly to upgrade for one reason or another.
In these cases they probably have sufficient in-house expertise to manage the software that runs on top of RHEL so they are not interested in that level of support, but they want to maintain a efficient division of labor were their in-house folks are able to concentrate on supporting the applications while they can depend on Redhat to help with the OS (software that is bundled with the OS counts as part of the OS in this scenario). That is it's more cost-effective for them to pay Redhat to support the OS stuff then have their admin's valuable time spent doing anything other then babysitting the software running on top of it.
That's just a guess. I know it's certainly true for many of the bigger corporations, but I wonder how true that is in general.