Django can move ahead and drop support for (honestly very outdated) Python versions like 2.5 and 2.6 so that newer Django releases can take advantage of additional features and nicer syntax in newer Python versions.
And companies stuck on python 2.6 because that's what's bundled in whatever version of RHEL can still get security patches to Django.
It's unreasonable to expect the (volunteer) django maintainers to indefinitely support every version of python or every version of Django.
And if you have no choice of upgrading to Django 1.5 yet, I'm sure you'd rather continue to get security patches until you can upgrade to 1.8.