Red Hat Enterprise Linux (and, therefore, CentOS, Scientific Linux, etc.) 6, which is the current version of the OS, comes with python 2.6. Version 5 of the OS, which is still supported, comes with python 2.4 and an optional python26 package, which installs python 2.6 in parallel with the default version. I assume that similar things are true for other long term support distributions.
In these distributions, using python 2.7 would require building it yourself and install it in parallel with the system-supplied version. This would add to the porting problems, that others have mentioned, the cost of maintaining python yourself. Thus, until these distributions update python to version 2.7, it is good that project maintainers provide support for these older versions.