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, using Linux based PCs.

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.