Developers keep developing, and they must keep up with Android versions, apps are often not backward compatible. Consider the video below. If you run 6, even though your hardware is quite capable, you might be out of luck.
https://www.youtube.com/watch?v=uaEnd4VzYZM
Boyue are investing the time to upgrade the OS. Onyx are choosing not to. That is Onyx's choice, and possibly a good one, in that they are investing resources to provide a solid product in terms of hardware and software. However...
Onyx are illegally not releasing their source code, so that the community cannot provide the OS upgrades Onyx choose not to provide. This is a huge black mark against Onyx, and certainly a reason that in the future I will at least consider other brands.