From the Ubuntu website:
OEMs submit systems to Canonical's testing facility where certification testing and validation is performed by Canonical engineers.
It is possible that System76 did not submit their systems for certification. If they did, it is possible that their systems were not up to Canonical's standards. One can only speculate as to why System76's systems are not Ubuntu-certified. For a more definite answer, you should contact System76.
A machine does not need to be certified for Ubuntu to work on it. From the Ubuntu website:
Ubuntu works on a range of hardware which has not been certified by the manufacturer. The Ubuntu community works together to test and report the range of systems that work with Ubuntu. You can see a list of tested systems with their results in the Ubuntu Friendly site.