By being mostly proprietary in new and important parts, built on top of an open foundation which itself is not the most valuable part for customers.
I suspect that key parts of Kai OS are proprietary, and the foundation left from Firefox OS is very basic. Important, but totally insufficient.
GerdaOS (LineageOS for KaiOS) allows people to import and run FirefoxOS packages without modification, though many apps will need some changes to allow it to work without a touchscreen.
The unofficial rumour mill says that with the extra investment from Google, KaiOS will probably shift to Chrome(ium) at some point in the future, but that's just a rumour and remains to be seen.
Mozilla spent years with dozens to hundreds of devs working on getting it to be snappier than every other low end phone, short of Google paying for this type of dev work on KaiOS, there will likely be nothing more than security patches and cosmetic changes to KaiOS.