Most of the time it's because of the drivers: not open source, not compatible with newer kernel, and not updated by the manufacturer.
This significantly shrinks the number of devices they support, but the devices that are supported have their lifespans increased significantly
Doesn't mean everything MTK does shines under the sun, but in Linux WiFi, they are probably the most open and well supported in the modern age.
6.x kernel is not expected to land anytime soon (late this year or early next year maybe?)
If you have a 6.x kernel, this is probably a fork of OpenWrt.