The real reason is simple -- the device in question (ASIX chipset-based dongle) was not certified to work with Windows RT.
In my preliminary testing, the device, for example, does not behave correctly when entering Connected Standby. If you're lucky, the vendor will make the changes needed to meet the Windows Hardware Certification Requirements and repost drivers. (Or if you're unlucky, the hardware just isn't cut out for the needs of today's devices.)
Given Windows RT cannot be configured to load test signed drivers (see: SecureBoot), I presume the driver floating around was provided by Microsoft to ASIX for internal dev/test use only. Given it was pulled and later blacklisted in 8.1, I can only assume (until my testing is complete) that the drivers sucked across one or more facets (e.g. quality, perf, and battery life).