Linux considers it a proprietary protocol [1] so Zigbee driver cannot be part of Linux kernel. Although Zigbee spec allows non-commercial individuals to freely use it, a commercial organization must be a member of the Zigbee Alliance in order to use Zigbee, which violates the GPL standard. [2]
At least the article speaks out "an open-source approach", let's see if things will get better.
[1]: https://www.kernel.org/doc/html/latest/networking/ieee802154...
[2]: https://archive.freaklabs.org/index.php/blog/zigbee/zigbee-l...