In addition to what others said, if you write code for python 3 making it compatible with python2.7 is relatively painless task (from my own experience) due to Python 2.7 having py3 functionality back ported.
Regarding python 2.6 this was discontinued back in 2013, as far as I know all redhat versions that used 2.6 are no longer supported unless you're paying for extended support. So I'm not convinced it is a good excuse.
Also if you use Java for example and you want to use JDK7 or JDK8 it doesn't matter that RH distro uses old version you'll find a proper RPM (or build one) to use it.
RHEL 6 uses python 2.6, and is supported till 2020 (not counting the extended support). Of course, if an app only needs a single host or two, its no problem to have an updated OS for that app. But certain app types, such as backup and monitoring apps, don't do any good unless they run on the majority of systems that a site has. In my case, I couldn't even try out several popular backup systems, as they either required Python 3, or had other library requirements that weren't available on the older enterprise systems.
In addition to what others said, if you write code for python 3 making it compatible with python2.7 is relatively painless task (from my own experience) due to Python 2.7 having py3 functionality back ported.
Regarding python 2.6 this was discontinued back in 2013, as far as I know all redhat versions that used 2.6 are no longer supported unless you're paying for extended support. So I'm not convinced it is a good excuse.
Also if you use Java for example and you want to use JDK7 or JDK8 it doesn't matter that RH distro uses old version you'll find a proper RPM (or build one) to use it.