I’m not sure, that’s a question for the Dev.
Some of the reason may be the hastle of rebranding, having two Openboards would be confusing so the fork would need to change names and icons and such. Some of it is also be this is for personal use, and we happen to find it, so they may not be interested in the expectation of maintaining it beyond their own useage. Some of it may be this is good enough, Openboard’s release cycle is pretty slow so the fork doesn’t need to be updated and released often, so an APK on Github downloaded twice a year is good enohgh for them.
It’s all about what you value, and supporting the things you love (or rely on, in a more utility sense). I’d value the speed, the lack of data collection that may be used against a user, the speed, the location options, and that same provider being in business for time to come. When I’d need a good VPN, nothing else will do. That seems worth the couple of USD per month to me, whether that VPN is for obscuring traffic I don’t want others to know about (whether it’d be because of those facilitating the connection or the other end of the connection).