According to https://opensource.stackexchange.com/questions/15792/debian-considers-the-ubuntu-font-family-non-free-why:

The problem appears to be around the requirement that modified versions which are not “substantially changed” must maintain the “Ubuntu” name - section 2.3 of the license:

    2.3 Modified Versions which are not Substantially Changed must be renamed to both

       1. retain the name of the Original Version and
       2. add additional naming elements to distinguish the Modified Version from the Original Version.

See Launchpad for a long discussion; the argument is that while the Debian Free Software Guidelines (DFSG) allow a copyright holder to require the name of derivatives be changed, the DFSG do not allow the copyright holder to specify how the name be changed. It is potentially worth noting that Fedora also considers the license non-free, but for different reasons.

TBH to me that doesn’t seem like strong enough grounds to not copy it manually to your Guix install, though I can see why distros might not want to package it. Canonical could probably have circumvented this issue by just using a more established license … one has to wonder how much of this is sheer ignorance, and how much is calculated discouragement of using their fonts on other distros.