[noggin]

I've been reading along and trying to follow the news on all these media players but something keeps bugging me that I'm not sure I've found an answer for.

What is in the Pi (or similar) which allows the full functionality of MVC 3D MKV files that isn't in other devices or video cards?

The Pi / Pi 2 has a "hardware" MVC (the 3D codec used alongside H264/AVC to encode 3D Blu-ray at full HD resolution) codec engineered into its GPU via firmware AND has Linux driver support for it, as well as Linux driver support for the GPU (and hardware support in the GPU) to output 1080/24p frame packed modes over HDMI. Two things that are both needed for 3D MVC decode AND full HD 3D output.

If sounds like it's something to do with firmware, allowing the HDMI on the Pi (or similar) to access the MVC functionality that is built into most HDMI ports now. Is that correct?

Firmware AND Linux driver support for 3D MVC decode and 3D HDMI Frame Packed output. (Not the same things - though often confused)

And if so, WTF can't/won't video card manufactures upgrade their firmware, even if only for specific models that aren't build for massing 4K 6 monitor gaming and the like?

Most video card manufacturers target Windows users - so their Windows drivers have 3D MVC decode support and support for frame packed 3D output. However these drivers may not be open to everyone to use (people behind WinDVD and PowerDVD have access to this stuff but it may required NDAs to access them (not compatible with Kodi GPL Open Source))

Most manufacturers only provide a subset of their cards' functionality via their Linux drivers. The same is true of ARM SoCs - where Android (and other non-Open Source) solutions are favoured.

Further, do the Pi manufactures have any ability to produce a PC video card with their functionality? I don't know all the logistics so just hoping to clarify some questions I have that I can relate to others who spend far more time frustrated with the 3D situation than they do looking into understanding it (old people!).

Not really. Remember the Pi Foundation is a charity geared up to educate kids about computers and coding. The Pi manufacturing is a separate division, but I don't see them taking on nVidia or AMD in the PC GPU market...