I think the reason is that it's basically only now that Linux distros are starting to ship Python 3 as the default. When RHEL, CentOS and Debian moves completely to Python 3, the rest of the world will follow.
Is the shipped default really so important, esp. for third-party software? E.g. for RHEL you get python3 packages through Red Hat Software Collections, with support and "intended for production use".
(It of course limits software that is to be shipped with the distro itself)
Those who use value a more rapid package update schedule shouldn't use those distros. One of their most salient features is the ancient software packages.
