Its fully DRM related.

But why does the HDMI forum not want a open source 2.1-compliant implementation?

To my knowledge they've never officially said but you can be sure that it has to do with Content Protection and that means DRM. An Open Source HDMI 2.1+ driver would make pirating much simpler, probably trivial and they don't want that.

It's possible anyway of course but there are a couple of hardware hoops to jump through and that's enough to keep most people from doing it.

Likely moreso that they're facing pressure from other competitors in the industry that see Steam and open source in general as a threat to their business model. The HDMI forum is made up of industry leaders, and naturally Microsoft and Sony are there.

https://hdmiforum.org/members/

Because that would open source certain implementations they want to hold captive.

It also enforces closed source drivers which can be shipped with spyware/crapware, further extending profits for companies... companies that happen to make up the HDMI Forum.

They charge a fee for access to the spec and maintain who can claim their products are HDMI compliant and require compliance testing on those products.

An open source implementation would make that spec public and strip a lot of control they hold.

Part of being open source is subsequent licensing. This would allow any others to piggyback and avoid the fee.